utils/git: use exact format for last_revision_*

[stek29]

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• 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. -- covered by existing tests
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• Have you successfully run brew tests with your changes locally?

---

Format for oneline can be overridden in user's gitconfig. If that's the
case, last_revision_commit_of_file won't work properly, and because of
that last_revision_of_file won't work at all.

Here's what I was getting when running brew tests

Failures:

  1) Git#last_revision_commit_of_file gives revision commit based on before_commit when it is not nil
     Failure/Error:
       expect(
         described_class.last_revision_commit_of_file(HOMEBREW_CACHE,
                                                     file,
                                                     before_commit: hash2),
       ).to eq(hash2)

       expected: "ae7a2b6"
            got: "\e[33mae7a2b6cdef0\e[m"

       (compared using ==)
     # ./test/utils/git_spec.rb:34:in `block (3 levels) in <top (required)>'
     # ./test/spec_helper.rb:106:in `block (2 levels) in <top (required)>'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:115:in `block in run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `loop'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec_ext/rspec_ext.rb:12:in `run_with_retry'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:33:in `block (2 levels) in setup'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `block (2 levels) in <top (required)>'

  2) Git#last_revision_commit_of_file gives last revision commit when before_commit is nil
     Failure/Error:
       expect(
         described_class.last_revision_commit_of_file(HOMEBREW_CACHE, file),
       ).to eq(hash1)

       expected: "6bec2de"
            got: "\e[33m6bec2dee633f\e[m"

       (compared using ==)
     # ./test/utils/git_spec.rb:28:in `block (3 levels) in <top (required)>'
     # ./test/spec_helper.rb:106:in `block (2 levels) in <top (required)>'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:115:in `block in run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `loop'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec_ext/rspec_ext.rb:12:in `run_with_retry'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:33:in `block (2 levels) in setup'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `block (2 levels) in <top (required)>'

  3) Git#last_revision_of_file returns last revision of file
     Failure/Error:
       expect(
         described_class.last_revision_of_file(HOMEBREW_CACHE,
                                               HOMEBREW_CACHE/file),
       ).to eq("blah")

       expected: "blah"
            got: ""

       (compared using ==)
     # ./test/utils/git_spec.rb:44:in `block (3 levels) in <top (required)>'
     # ./test/spec_helper.rb:106:in `block (2 levels) in <top (required)>'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:115:in `block in run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `loop'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:104:in `run'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec_ext/rspec_ext.rb:12:in `run_with_retry'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-retry-0.5.6/lib/rspec/retry.rb:33:in `block (2 levels) in setup'
     # ./vendor/bundle/ruby/2.3.0/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in `block (2 levels) in <top (required)>'

Finished in 1.35 seconds (files took 1.69 seconds to load)
17 examples, 3 failures, 1 pending

Failed examples:

rspec ./test/utils/git_spec.rb:33 # Git#last_revision_commit_of_file gives revision commit based on before_commit when it is not nil
rspec ./test/utils/git_spec.rb:27 # Git#last_revision_commit_of_file gives last revision commit when before_commit is nil
rspec ./test/utils/git_spec.rb:43 # Git#last_revision_of_file returns last revision of file

I have abbrev length set to 12, and oneline formatting is more colorful.

So, instead of relying on overrideable format, it should rather specify
"--format=%h" to get only hash, and speciy --abbrev=7 to force abbrev
length to be 7

After this commit all tests are passing.

[stek29]

==> Downloading https://homebrew.bintray.com/bottles/openssl@1.1-1.1.0g_1.sierra.bottle.tar.gz
curl: (6) Could not resolve host: homebrew.bintray.com
Error: Failed to download resource "openssl@1.1"

I guess I'd edit commit message a bit to trigger re-buid.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @stek29!