subrepo: don't commit in subrepo if it's clean
If a subrepo has changed relative to the outer repo's substate, but the
subrepo itself is clean, we don't need to commit in the subrepo.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state