contrib/subtree: unwrap tag refs

contrib/subtree: unwrap tag refs

If a subtree was added using a tag ref, the tag ref is stored in
the subtree commit message instead of the underlying commit's ref.
To split or push subsequent changes to the subtree, the subtree
command needs to unwrap the tag ref.  This patch makes it do so.

The problem was described in a message to the mailing list from
Junio C Hamano dated 29 Apr 2014, with the subject "Re: git subtree
issue in more recent versions". The archived message can be found
at <http://comments.gmane.org/gmane.comp.version-control.git/247503>.

Signed-off-by: Rob Mayoff <mayoff@dqd.com>
Signed-off-by: Jeff King <peff@peff.net>