Expand Repository Compare tests

We currently test the Repository Compare API for commits. However the API can also be used for Branches or Tags.

After clarification with Gitaly and Source Code teams (see !330 (merged)), we learned that Repository Compare functionality works the same for commits, tags and branches. Thus the issue is to supersede the current Repository Compare Commits test with Repository Compare Branches test.

We should also look into adding an equivalent Web test for the compare page