Fix a bug in "hg" module so that `pull` is not set to a version.

There is a bug in the `hg` module where if a `tag` is used it won't
properly be able to update to it.

This problem is exhibited on repository where a tag is moved from an
older commit to a newer commit.

library/source_control/hg

@@ -167,7 +167,7 @@ class Hg(object):
 
     def pull(self):
         return self._command(
-            ['pull', '-r', self.revision, '-R', self.dest, self.repo])
+            ['pull', '-R', self.dest, self.repo])
 
     def update(self):
         return self._command(['update', '-R', self.dest])