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

mercurial/commands.py

@@ -4559,8 +4559,13 b' def resolve(ui, repo, *pats, **opts):'
     all, mark, unmark, show, nostatus, remerge = \
         [opts.get(o) for o in flaglist]
 
-    if len(list(filter(None, [show, mark, unmark, remerge]))) > 1:
-        raise error.Abort(_("too many options specified"))
+    actioncount = len(list(filter(None, [show, mark, unmark, remerge])))
+    if actioncount > 1:
+        raise error.Abort(_("too many actions specified"))
+    elif (actioncount == 0
+          and ui.config('commands', 'resolve.explicit-re-merge')):
+        hint = _('use --mark, --unmark, --list or --re-merge')
+        raise error.Abort(_('no action specified'), hint=hint)
     if pats and all:
         raise error.Abort(_("can't specify --all and patterns"))
     if not (all or pats or show or mark or unmark):

mercurial/configitems.py

@@ -193,6 +193,9 b" coreconfigitem('commands', 'grep.all-fil"
 coreconfigitem('commands', 'resolve.confirm',
     default=False,
 )
+coreconfigitem('commands', 'resolve.explicit-re-merge',
+    default=False,
+)
 coreconfigitem('commands', 'resolve.mark-check',
     default='none',
 )

mercurial/help/config.txt

@@ -442,6 +442,11 b' effect and style see :hg:`help color`.'
     Confirm before performing action if no filename is passed.
     (default: False)
 
+``resolve.explicit-re-merge``
+    Require uses of ``hg resolve`` to specify which action it should perform,
+    instead of re-merging files by default.
+    (default: False)
+
 ``resolve.mark-check``
     Determines what level of checking :hg:`resolve --mark` will perform before
     marking files as resolved. Valid values are ``none`, ``warn``, and

tests/test-resolve.t

@@ -448,12 +448,22 b' Testing the --re-merge flag'
   U file1
   R file2
   $ hg resolve --mark --re-merge
-  abort: too many options specified
+  abort: too many actions specified
   [255]
   $ hg resolve --re-merge --all
   merging file1
   warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
   [1]
+Explicit re-merge
+  $ hg resolve --unmark file1
+  $ hg resolve --config commands.resolve.explicit-re-merge=1 --all
+  abort: no action specified
+  (use --mark, --unmark, --list or --re-merge)
+  [255]
+  $ hg resolve --config commands.resolve.explicit-re-merge=1 --re-merge --all
+  merging file1
+  warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
+  [1]
 
   $ cd ..