ostbuild: Add 'tag' concept to manifest, improve git mirroring

ostbuild: Add 'tag' concept to manifest, improve git mirroring

We should explicitly distinguish between the case where we have a git
branch we're following, versus an immutable tag.  In the latter case,
we can entirely avoid running 'git fetch' for it once we have it.
This is a noticeable speedup in our current scenario of pinning WebKit
to a tag.

The git mirroring code now has a --fetch-skip-secs=X option which
allows us to basically run it in a loop, without hitting remote git
repositories too often.