merge: add commands.merge.require-rev to require an argument to hg merge
This is related to commands.rebase.requiredest, commands.update.requiredest, and
commands.push.require-revs. Since it isn't really a "destination", I went with
require-rev to be similar to push's require-revs.
Differential Revision: https://phab.mercurial-scm.org/D7620
$ 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