mq: silence spurious output.

When using a versioned patch repository, you would get a spurious
warning when deleting and adding the same patch.

Before:

  $ hg qdelete --keep 3.diff
  $ hg qimport --existing 3.diff
  adding 3.diff to series file
  3.diff already tracked!

After:

  $ hg qdelete --keep 3.diff
  $ hg qimport --existing 3.diff
  adding 3.diff to series file

hgext/mq.py

@@ -479,7 +479,7 b' class queue(object):'
         if self.added:
             qrepo = self.qrepo()
             if qrepo:
-                qrepo[None].add(self.added)
+                qrepo[None].add(f for f in self.added if f not in qrepo[None])
             self.added = []
 
     def removeundo(self, repo):

tests/test-mq-qdelete.t

@@ -150,3 +150,15 b' Delete the same patch twice in one comma'
   series
   status
 
+qdel -k X && hg qimp -e X used to trigger spurious output with versioned queues
+
+  $ hg init --mq
+  $ hg qimport -r 3
+  $ hg qpop
+  popping 3.diff
+  patch queue now empty
+  $ hg qdel -k 3.diff
+  $ hg qimp -e 3.diff
+  adding 3.diff to series file
+  $ hg qfinish -a
+  no patches applied