project: add ManifestProject.Sync()

Move the logic to sync a ManifestProject out of subcmds/init.py

Change-Id: Ia9d00f3da1dc3c5dada84c4d19cf9802c2346cb0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334140
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>