filterfiles: Search as long as the target is a prefix of current.

filterfiles was failing to find files for directory arguments if
another file existed that started with the directory name and
sorted earlier. For example, a manifest of ('foo.h', 'foo/foo')
would cause filterfiles('foo') to return nothing. This resolves
issue  #294.

mercurial/dirstate.py

@@ -285,8 +285,9 b' class dirstate(object):'
                 continue
             while bs < blen:
                 s = b[bs]
-                if len(s) > len(x) and s.startswith(x) and s[len(x)] == '/':
-                    ret[s] = self.map[s]
+                if len(s) > len(x) and s.startswith(x):
+                    if s[len(x)] == '/':
+                        ret[s] = self.map[s]
                 else:
                     break
                 bs += 1