merge: deprecate the --force option

The --force option in merge does not make what people think it does so
it may not be visible to everyone.

I have local changes and want to pull one's changes which made 2 heads.
The --force option in help says

 -f --force      force a merge with outstanding changes

so I can expect that I can use it to force the merge and commit it in my
local repository without taking my local changes into account. But
merging with -f keeps local changes and "add" them: they must be
committed or reverted before doing the merge commit. The merge -f cannot
be reverted so it leads my repository in a bad state: cannot commit
merge and don't want to revert/commit local changes yet.

Message in help have been updated to emphasize the fact that local
changes are included in the merge.

mercurial/commands.py

@@ -4158,7 +4158,8 b' def manifest(ui, repo, node=None, rev=No'
     fm.end()
 
 @command('^merge',
-    [('f', 'force', None, _('force a merge with outstanding changes')),
+    [('f', 'force', None,
+      _('force a merge including outstanding changes (DEPRECATED)')),
     ('r', 'rev', '', _('revision to merge'), _('REV')),
     ('P', 'preview', None,
      _('review revisions to merge (no merge is performed)'))