match: add basic wrapper for boolean function

This serves as a generic wrapper for fileset predicates. In future patches,
a fileset expression will be mapped to a tree of matchers for a better support
of match attributes such as visitdir(). For example,

  $ hg debugwalk -v 'set:contrib/** and binary()'
  * matcher:
  <intersectionmatcher
    m1=<patternmatcher patterns='(?:contrib/.*$)'>,
    m2=<predicatematcher pred=binary>>
  ...

mercurial/match.py

@@ -375,6 +375,20 b' class nevermatcher(basematcher):'
     def __repr__(self):
         return r'<nevermatcher>'
 
+class predicatematcher(basematcher):
+    """A matcher adapter for a simple boolean function"""
+
+    def __init__(self, root, cwd, predfn, predrepr=None, badfn=None):
+        super(predicatematcher, self).__init__(root, cwd, badfn)
+        self.matchfn = predfn
+        self._predrepr = predrepr
+
+    @encoding.strmethod
+    def __repr__(self):
+        s = (stringutil.buildrepr(self._predrepr)
+             or pycompat.byterepr(self.matchfn))
+        return '<predicatenmatcher pred=%s>' % s
+
 class patternmatcher(basematcher):
 
     def __init__(self, root, cwd, kindpats, ctx=None, listsubrepos=False,