install.rb: stop early when there are no formulae to be installed

[maxim-belkin]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

---

This PR makes so that Homebrew exits early when there are no formulae to be installed.

I've added some debugging info to demonstrate what this PR does:

BEFORE:

 $ brew install gdm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:show_summary_heading, :show_header]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:build_from_source, :force_bottle]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:ignore_deps, :only_deps, :interactive, :git]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:verbose, :debug, :quieter, :link_keg]
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: install
Warning: gdm 1.4 is already installed
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: perform_preinstall_checks
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_ppc
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_writable_install_location
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_development_tools
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_cellar
$

AFTER

$ brew install gdm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:show_summary_heading, :show_header]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:build_from_source, :force_bottle]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:ignore_deps, :only_deps, :interactive, :git]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:verbose, :debug, :quieter, :link_keg]
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: install
Warning: gdm 1.4 is already installed
$

This PR does not break building multiple formulae:

$ brew install gdbm gdm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:show_summary_heading, :show_header]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:build_from_source, :force_bottle]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:ignore_deps, :only_deps, :interactive, :git]
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: mode_attr_accessor
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: names = [:verbose, :debug, :quieter, :link_keg]
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: install
Warning: gdm 1.4 is already installed
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: perform_preinstall_checks
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_ppc
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_writable_install_location
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_development_tools
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: check_cellar
DEBUG [/usr/local/Homebrew/Library/Homebrew/cmd/install.rb]: install_formula
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: initialize
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: formula = gdbm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: prelude
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: verify_deps_exist
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: compute_dependencies
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: expand_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: runtime_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: formula = gdbm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: check_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: req_map = {}
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: expand_dependencies
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: deps = []
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: lock
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: check_install_sanity
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: install
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: check_conflicts
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: compute_dependencies
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: expand_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: runtime_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: formula = gdbm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: check_requirements
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: req_map = {}
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: expand_dependencies
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: deps = []
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: install_dependencies
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: deps = []
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: display_options
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: formula = gdbm
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: pour
==> Downloading https://homebrew.bintray.com/bottles/gdbm-1.13.sierra.bottle.tar.gz
Already downloaded: /Users/mbelkin/Library/Caches/Homebrew/gdbm-1.13.sierra.bottle.tar.gz
==> Pouring gdbm-1.13.sierra.bottle.tar.gz
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: puts_requirement_messages
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: finish
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: install_plist
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: link
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: keg = /usr/local/Cellar/gdbm/1.13
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: fix_dynamic_linkage
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: keg = /usr/local/Cellar/gdbm/1.13
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_bottle?
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: post_install
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: caveats
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: summary
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: build_time
🍺  /usr/local/Cellar/gdbm/1.13: 19 files, 554.4KB
DEBUG [/usr/local/Homebrew/Library/Homebrew/formula_installer.rb]: unlock

[ilovezfs]

exists early

Is that before or after the Big Bang?

[MikeMcQuaid]

Makes sense to me!

[MikeMcQuaid]

Thanks again @maxim-belkin!

[maxim-belkin]

Is that before or after the Big Bang?

🙈🙄

s/exists/exits/