merge: change default of hg.merge()'s "force" argument from None to False

The argument is only passed to `mergemode.update()`, and that function
treats `None` just like `False`, so `False` seems clearer.

Differential Revision: https://phab.mercurial-scm.org/D8166

mercurial/hg.py

@@ -1137,7 +1137,7 b' def updatetotally(ui, repo, checkout, br'
 
 
 def merge(
-    repo, node, force=None, remind=True, mergeforce=False, labels=None,
+    repo, node, force=False, remind=True, mergeforce=False, labels=None,
 ):
     """Branch merge with node, resolving changes. Return true if any
     unresolved conflicts."""