formula: enable Git repository extension for buildpath/testpath

[SeekingMeaning]

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This will allow instances of git rev-parse HEAD to be replaced with buildpath.git_head or testpath.git_head

Note: derived paths such as buildpath.parent.git_head or (testpath/"foobar").git_head won't work as GitRepositoryExtension is only enabled for buildpath and testpath specifically.

References to rev-parse in homebrew-core: https://github.com/Homebrew/homebrew-core/search?q=rev-parse

[BrewTestBot]

Review period will end on 2020-12-28 at 22:43:39 UTC.

[reitermarkus]

Suggested change

	Utils.popen_read("git", "rev-parse", "--short#{"=" if length.present?}#{length}",
	Utils.popen_read("git", "rev-parse", "--short#{length.presence&.prepend("=")}",

[reitermarkus]

This is currently a private API.

[MikeMcQuaid]

👍🏻 from me if/when the docs are updated to clarify this is no longer a private API.

[BrewTestBot]

Review period ended.

[SeekingMeaning]

Closing since it seems strange not to be able to do buildpath.parent.git_head

I think something like Utils.git_head(buildpath.parent) would be more appropriate