[IMP] runbot_merge: add number to PR search view

The number is probably the most common search criteria for PRs (to
track their status / issues). Having to go through custom filters to
find one is a pain in the ass.

Already done live by editing the view, but means it's getting lost
every time the module gets updated.

closes #73

runbot_merge/views/mergebot.xml

@@ -72,6 +72,7 @@
                     name="open" string="Open"
                     domain="[('state', 'not in', ['merged', 'closed'])]"
                 />
+                <field name="number"/>
                 <field name="author"/>
                 <field name="label"/>
                 <field name="target"/>