match: add match.ispartial()

match.ispartial() will return the opposite of match.always() in core, but this
function will be extensible by extensions to produce another result even
if match.always() will be untouched.

This will be useful for narrowhg, where ispartial() will return False even if
the match won't always match. This would happen in the case where the only
time the match function is False is when the path is outside of the narrow
spec.

mercurial/localrepo.py

@@ -1377,7 +1377,7 b' class localrepository(object):'
             wctx = self[None]
             merge = len(wctx.parents()) > 1
 
-            if not force and merge and not match.always():
+            if not force and merge and match.ispartial():
                 raise util.Abort(_('cannot partially commit a merge '
                                    '(do not specify files or patterns)'))
 

mercurial/match.py

@@ -186,6 +186,14 b' class match(object):'
         - optimization might be possible and necessary.'''
         return self._always
 
+    def ispartial(self):
+        '''True if the matcher won't always match.
+
+        Although it's just the inverse of _always in this implementation,
+        an extenion such as narrowhg might make it return something
+        slightly different.'''
+        return not self._always
+
     def isexact(self):
         return self.matchfn == self.exact