Branch discrimination for GitPollers

[gracinet]

Somehow, changes in master seem to trigger builds on other branches (seen on Odoo, but would probably be the same for other repos):

http://buildbot.anybox.fr/builders/odoo-80-postgresql-9.3/builds/560
http://buildbot.anybox.fr/builders/odoo-80-postgresql-9.3/builds/569

(that's before first tests of #1 on this buildbot)

[gracinet]

Well ok, it's been working fine since then