mirror_pull: parse 'git fetch --porcelain' instead of human readable output

[tycho]

I was surprised to see that the pull mirroring mechanism is trying to parse the human-readable git fetch output instead of using the machine-readable git fetch --porcelain or an equivalent approach.

This change makes the parsing less fragile since the underlying command now has a predictable output format.

I do notice that git fetch --porcelain was only added in Git 2.41.0 (~8 months ago?), according to the release notes. So I might need to add a fallback mechanism to use the existing parseRemoteUpdateOutput if we're using an old git version, with a new feature flag added here?

Also, an alternative approach would be to do something like... record the output of git for-each-ref --format='%(refname) %(objectname)' before and after the git fetch invocation and figure out the differences based on that, but I'm open to suggestions.

[lunny]

Since it requires a high version of git, a version feature check and fallback is necessary.