remote: Refactor UpdateRemoteMirror test to use WriteCommit

In order to set up test data for the UpdateRemoteMirror RPC we use gitaly-git2go to write various commits into the repository. Besides being slow, it also complicates the test setup.

Refactor the code to just use gittest.WriteCommit(). This reduces the overall time required to execute these tests from 5.3s to 2.6s on my machine. Furthermore, I hope that this may fix flakiness observed in this test. While I haven't really found out about the root cause of those flakes, we can see that it takes a long time to execute all of the gitaly-git2go invocations on FIPS runners.