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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif