fetch: Add ability to fast-forward existing bitbake / meta layer git repos.

This is a bit conservative in that we only fast forward and do nothing
if the git repo and the LAYERS file disagree on the branch name.
fetch.sh: Don't pushd/popd unnecessarily for metas dir.
fetch.sh: Quiet down 'git status' when detecting changes in repos.
Merge branch 'master' into dom0