Add git rev-parse completions

[rasitakyol]

Summary

• add a git rev-parse signature with an optional variadic revision argument
• suggest local branches, remote branches, commits, and tags
• preserve the existing handwritten git.json signature structure

Closes #292.
Closes warpdotdev/warp#12319.

Testing

• ./script/presubmit (format, clippy, 76 tests, and doc tests)
• manual end-to-end verification in a local Warp build using this checkout as the command-signatures dependency

Screenshot

The completion menu shows the current branch, local and remote branches, a commit, and a tag for git rev-parse.

[oz-for-oss]

@rasitakyol

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

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[oz-for-oss]

@rasitakyol

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

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[rasitakyol]

Addressed the same-repository issue requirement: I created #292 and linked it from this PR with Closes #292. The issue is now awaiting the maintainer-applied ready-to-implement label; once that is set, I will re-trigger /oz-review.