outgoing: fix possible filtering crash in outgoing (issue3814)

If there is no outgoiing changesets but we have filtered revision in outgoing.excluded
We run into a filtering related crash. The excluded revision should not be there
in the first place but discovery need cleanup in default, not stable.

--HG--
branch : stable
extra : source : e7b89b5127c2607bcaedf7c2fb0ea525df526ea6