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.