revset: don't recreate matcher for every revision

The matcher variable 'm' in checkstatus() is reset to None on each
call, so the caching of the matcher no longer happens as it was
intended. This seems to be a regression in ed7b674824a3 (revset: added
lazyset implementation to checkstatus, 2014-01-03).

Fix by moving the cached matcher into the enclosing function so it's
actually cached across calls. This speeds up

  hg log -r 'modifies(mercurial/context.py)' >/dev/null

from 7.5s to 4s.

Also see similar fix in f2aeff8a87b6 (revset: avoid recalculating
filesets, 2014-10-22).

mercurial/revset.py

@@ -526,14 +526,15 b' def bundle(repo, subset, x):'
 def checkstatus(repo, subset, pat, field):
     hasset = matchmod.patkind(pat) == 'set'
 
+    mcache = [None]
     def matches(x):
-        m = None
-        fname = None
         c = repo[x]
-        if not m or hasset:
-            m = matchmod.match(repo.root, repo.getcwd(), [pat], ctx=c)
-            if not m.anypats() and len(m.files()) == 1:
-                fname = m.files()[0]
+        if not mcache[0] or hasset:
+            mcache[0] = matchmod.match(repo.root, repo.getcwd(), [pat], ctx=c)
+        m = mcache[0]
+        fname = None
+        if not m.anypats() and len(m.files()) == 1:
+            fname = m.files()[0]
         if fname is not None:
             if fname not in c.files():
                 return False