ruby: Reduce usage of Git commands

While implementing !4557 (merged) I noticed that while we still have some code around to execute e.g. git-fetch(1), that code is not in fact used anymore. This MR removes unused code to interact with remote repos and thus gets rid of the last invocation of git-fetch(1). Furthermore, I'm converting the last callsite of git-symbolic-ref(1) to use Rugged instead to also get rid of that command.

With these changes we only have a single callsite of Git left in the Ruby sidecar, which is the use of git-update-ref(1). Unfortunately, that one cannot go away just yet given that Rugged doesn't expose git_reference_create_matching().