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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2