Clean up Gitlab::Git::Commit.find_all

What does this MR do?

• Consolidate Gitlab::Git::Repository#find_commits and Gitlab::Git::Commit.find_all
• Remove unused 'contains' option from commit finder

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

Why was this MR needed?

Clean-up to make migrating to Gitaly easier.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?