outgoing: move filtering logic in its own function
This move code dedicated to a single purpose together and make the main code
simpler. Right when we are getting ready to make it more complex :-D
Differential Revision: https://phab.mercurial-scm.org/D10382
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state