resolve: add config to make hg resolve not re-merge by default
Before this, calling 'hg resolve' with neither -m, -u or -l will
re-merge. This is highly error prone (it's easy to forget to forget a
-m), and pretty bad when it happens (many people have no idea 'hg
resolve' can re-merge, and end up redoing the work of resolving all
the conflicts, because they have no idea there was a backup of their
work).
Differential Revision: https://phab.mercurial-scm.org/D4379