utils: add git_repository

[SeekingMeaning]

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(Supersedes #10149)

This PR adds utils/git_repository.rb which provides the publicly available methods Utils.git_head and Utils.git_head_short that can be used in formulae such as h2spec. I added these methods separately from utils/git.rb so that it does not have to be excluded from the Style/Documentation rubocop

Before:

commit = Utils.safe_popen_read("git", "rev-parse", "--short", "HEAD").chomp

After:

commit = Utils.git_short_head(buildpath)

Note: git rev-parse --short does not return a constant-length string—see https://stackoverflow.com/q/44286795

[BrewTestBot]

Review period will end on 2021-01-08 at 05:42:47 UTC.

[MikeMcQuaid]

Thanks @SeekingMeaning!

[MikeMcQuaid]

I know you didn't write this but: if this could be refactored to not be unless .. && and instead an if I'd be grateful 🙇🏻. For bonus points in another PR: I'd love a rubocop that checks for unless .. && and unless ... ||. rubocop/rubocop#5400 was a first attempt for this in RuboCop itself but I think it'd be fine for this to just be a local cop for Homebrew until/if you're able to upstream it.

[MikeMcQuaid]

Can docs be added (as these are public methods) instead of this being added to todo?

[SeekingMeaning]

How do I fix this style error?

Library/Homebrew/utils/git_repository.rb:4:1: C: Missing top-level module documentation comment.
module Utils
^^^^^^

What about utils/popen and utils/fork (which both extend Utils)? Would adding top-level documentation comments cause conflicts?

[MikeMcQuaid]

How do I fix this style error?

Ah, yes, I see. Would need to perhaps define that module in utils.rb and document it? I think it's fine to punt on that, then.

[BrewTestBot]

Review period ended.