remotefilelog: fix bug in maybesparsematch returning alwaysmatcher

The description of the method says that it should return None if sparse is not
used in this repository; since sparse.matcher() returns alwaysmatcher if sparse
is not enabled, I'm using that as the signal to return None here to preserve the
previous behavior.

Differential Revision: https://phab.mercurial-scm.org/D5487

hgext/remotefilelog/shallowrepo.py

@@ -143,8 +143,13 b' def wraprepo(repo):'
             sparse repository.
             '''
             if revs:
-                return sparse.matcher(repo, revs=revs)
-            return sparse.matcher(repo)
+                ret = sparse.matcher(repo, revs=revs)
+            else:
+                ret = sparse.matcher(repo)
+
+            if ret.always():
+                return None
+            return ret
 
         def file(self, f):
             if f[0] == '/':