Remove Rugged::Walker from Repository#log

What does this MR do?

Removes supposedly-fast Git commit listing code from Gitlab::Git::Repository#log. For some reason this 'faster' code path is now a lot slower.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/42089#note_54969778.

In short, gitlab_git!69 (merged) introduced a special code path based on Rugged::Walker that was faster than the default (git log) at the time. For some reason git log is much faster for us now. Removing the Rugged::Walker path makes things simpler and faster.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42089.