Implement porcelain mode for git-fetch(1)

In order to implement Update references atomically in FetchRemote (gitaly#3780 - closed), we need to implement a mode in git-fetch(1) that gives us machine-parseable output of references that change during a fetch. This mode needs to work with --dry-run so that we can fetch into a quarantine directory, parse the references, but then ultimately handle the reference update ourselves.

We should take git status --porcelain as inspiration here.