subrepo: clarify comments in dirty() methods

Just a little change to bring the comments in the dirty() methods of the
various subrepo classes into a uniform structure. This clarifies the
meaning of the states checked.

mercurial/subrepo.py

@@ -398,8 +398,8 b' class hgsubrepo(abstractsubrepo):'
         if r == '' and not ignoreupdate: # no state recorded
             return True
         w = self._repo[None]
-        # version checked out changed?
         if w.p1() != self._repo[r] and not ignoreupdate:
+            # different version checked out
             return True
         return w.dirty() # working directory changed
 
@@ -744,8 +744,8 b' class gitsubrepo(abstractsubrepo):'
                                (revision, self._relpath))
 
     def dirty(self, ignoreupdate=False):
-        # version checked out changed?
         if not ignoreupdate and self._state[1] != self._gitstate():
+            # different version checked out
             return True
         # check for staged changes or modified files; ignore untracked files
         out, code = self._gitdir(['diff-index', '--quiet', 'HEAD'])