Optimize return value of util._matcher for common command line case

This will trigger every time somebody runs something like "hg diff"
or "hg status" without any arguments.

The important part here is returning util.always as the match function,
which is a much simpler (and faster) function than the usual return
value, and allows other code to just skip the filtering if it knows
all files will match.

mercurial/util.py

@@ -418,6 +418,10 b' def _matcher(canonroot, cwd, names, inc,'
     - a bool indicating if any patterns were passed in
     """
 
+    # a common case: no patterns at all
+    if not names and not inc and not exc:
+        return [], always, False
+
     def contains_glob(name):
         for c in name:
             if c in _globchars: return True