subrepo: explicitly request clean and unknown files in status for git's add

No behavior changes here since gitsubrepo.status() doesn't currently populate
clean, and ignores whether unknown files were actually requested.  But this is
in line with other calls to status, and should avoid future surprises.

mercurial/subrepo.py

@@ -1528,7 +1528,8 b' class gitsubrepo(abstractsubrepo):'
             return []
 
         (modified, added, removed,
-         deleted, unknown, ignored, clean) = self.status(None)
+         deleted, unknown, ignored, clean) = self.status(None, unknown=True,
+                                                         clean=True)
 
         tracked = set()
         # dirstates 'amn' warn, 'r' is added again