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
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