Implement branch operations RPCs

Closes https://gitlab.com/gitlab-org/gitaly/issues/498

I'd suggest reviewing each commit on its own, but as a summary:

• one commit to change commit parsing to something like diff parsing, because it was tightly coupled to the commit package and getting commit(s) had to be used in a sending context (not Go context :P). Now we get GitCommit objects easily to be used in tests and whatnot.
• one commit to retire CommitsEqual in favor of require.Equal as the latter does a better job at diffing.
• one commit that does the implementation pointed at by the title.