sync: warn if not tracking a branch

Since tracking a branch prevents repo from updating, make sure we
warn people about the situation when using `repo sync`.

Bug: https://crbug.com/gerrit/11045
Change-Id: I966513f510827cc93194f8df176c6745946bd739
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258892
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>