debrew: fix irb start

[bayandin]

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---

irb doesn't start from debug menu with an error FrozenError: can't modify frozen #<Class:#<Array:0x00007fd4b48840b0>>.
irb#setup method defaults argv to ::ARGV and tries to shift it, which fails for out case when ::ARGV is frozen.

This PR defaults argv to an empty array (also we don't really want to pass brew cli arguments toirb).

The whole log looks like this:

brew install -s -d hello
...
RuntimeError: boom
1. raise
2. ignore
3. backtrace
4. irb
5. shell
Choose an action: 4
When you exit this IRB session, execution will continue.

WARNING: This version of ruby is included in macOS for compatibility with legacy software. 
In future versions of macOS the ruby runtime will not be available by 
default, and may require you to install an additional package.

/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/irb/init.rb:134:in `shift'
FrozenError: can't modify frozen #<Class:#<Array:0x00007fd4b48840b0>>
1. raise
2. backtrace
3. shell
Choose an action: 2
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/irb/init.rb:134:in `shift'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/irb/init.rb:134:in `parse_opts'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/irb/init.rb:27:in `setup'
/usr/local/Homebrew/Library/Homebrew/debrew/irb.rb:14:in `start_within'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:127:in `block (5 levels) in debug'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mutex_m.rb:78:in `synchronize'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mutex_m.rb:78:in `mu_synchronize'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:127:in `block (4 levels) in debug'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:17:in `raise'
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/hello.rb:22:in `install'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:24:in `block in install'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:94:in `debrew'
/usr/local/Homebrew/Library/Homebrew/debrew.rb:24:in `install'
/usr/local/Homebrew/Library/Homebrew/build.rb:174:in `block (2 levels) in install'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1187:in `block in brew'
/usr/local/Homebrew/Library/Homebrew/formula.rb:2174:in `block (2 levels) in stage'
/usr/local/Homebrew/Library/Homebrew/utils.rb:494:in `with_env'
/usr/local/Homebrew/Library/Homebrew/formula.rb:2173:in `block in stage'
/usr/local/Homebrew/Library/Homebrew/resource.rb:112:in `block in unpack'
/usr/local/Homebrew/Library/Homebrew/resource.rb:198:in `block in mktemp'
/usr/local/Homebrew/Library/Homebrew/mktemp.rb:57:in `block in run'
/usr/local/Homebrew/Library/Homebrew/mktemp.rb:57:in `chdir'
/usr/local/Homebrew/Library/Homebrew/mktemp.rb:57:in `run'
/usr/local/Homebrew/Library/Homebrew/resource.rb:197:in `mktemp'
/usr/local/Homebrew/Library/Homebrew/resource.rb:107:in `unpack'
/usr/local/Homebrew/Library/Homebrew/resource.rb:82:in `stage'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/forwardable.rb:230:in `stage'
/usr/local/Homebrew/Library/Homebrew/formula.rb:2153:in `stage'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1180:in `brew'
/usr/local/Homebrew/Library/Homebrew/build.rb:142:in `block in install'
/usr/local/Homebrew/Library/Homebrew/utils.rb:494:in `with_env'
/usr/local/Homebrew/Library/Homebrew/build.rb:137:in `install'
/usr/local/Homebrew/Library/Homebrew/build.rb:226:in `<main>'
1. raise
2. backtrace
3. shell
...

[MikeMcQuaid]

Makes sense to me, thanks for the detailed explanation and fix!