fetch: refactor code that prints reference updates

In the context of Implement porcelain mode for git-fetch(1) (#147 - closed) we want to introduce a new machine-parseable porcelain mode for git-fetch(1). I had a very hard time understanding the current way displaying of reference updates was implemented though as the logic is all over the place and makes heavy use of global variables. I thus decided to first create preliminary patch series that refactors the code to centralize that logic into a single place first.