commit: allow to close branch when committing change over a closed head
Otherwise, an explicit other commit become necessary, which seems both silly and
verbose.
This is useful when merging closed heads on the same branches, for example when
merging multiple repositories together.
$ 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