subrepo: return only the manifest keys from hgsubrepo.files()

This is in line with the other subrepo classes (i.e. only the filenames are
returned).  Archive iterates over the manifest keys, and since subrepo archive()
uses this method, it does too now.  This will allow largefiles to override its
manifest keys to present the largefiles instead of the standins.  Once in place,
we shouldn't need the copy/paste overrides of archive and subrepo archive in
largefiles, that don't quite behave like the core methods they are overriding.

mercurial/subrepo.py

@@ -836,7 +836,7 b' class hgsubrepo(abstractsubrepo):'
     def files(self):
         rev = self._state[1]
         ctx = self._repo[rev]
-        return ctx.manifest()
+        return ctx.manifest().keys()
 
     def filedata(self, name):
         rev = self._state[1]