Use git rev-parse instead of gogit.Repository.Head() to find out head branch & sha

[maciejp-ro]

Summary

When working in a git worktree rather than a full clone, plural build fails:

$ plural --debug build --only console
ERROR: 2023/10/03 14:11:38 validation.go:196: reference not found
2023/10/03 14:11:38 Failed to get git information: Could not compare current workspace to origin. Do you have an `origin` remote configured, or does your repo not have an initial commit?

I managed to track it down to gogit.Repository.Head() function: it seems to try to directly access objects in .git/. In worktree, .git is a file with reference to a parent repository, so there's no .git/HEAD.

This change uses actual git CLI instead of an incomplete reimplementation to get HEAD SHA and branch name. This makes Plural CLI work in a git worktree.

Some unrelated unit tests are failing, haven't touched these.

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Checklist

• If required, I have updated the Plural documentation accordingly.
• I have added tests to cover my changes.
• I have added a meaningful title and summary to convey the impact of this PR to a user.
• I have added relevant labels to this PR to help with categorization for release notes.

[michaeljguarino]

This looks should work, we actually used rev-parse prior and I think i flipped it to leverage a typed api. If it breaks some workflows though, should definitely revert.