Remove Gitlab::Popen dependency from lib/gitlab/git

What does this MR do?

Removes an unnecessary dependency of Gitlab::Git on Gitlab::Popen. This matters because lib/gitlab/git is vendored into gitaly-ruby; this change avoids having to vendor unnecessary files into gitaly-ruby.

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Why was this MR needed?

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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