[FIX] forwardport: fix reminder cron to avoid multiple messages

Because the reminder cron uses groupby to "merge" open PRs related to the
same source and send a single message for all of them (e.g. PR 6548
forward-ported to 6587 and 6591 should have a single reminder message per
day not one per descendant), the PRs with the same source need to be
consecutive in the search sequence.

However there was no order specified so the search would yield PRs in id
order or something, and if there happened to be an other forward-port PR
inbetween the descendants of the original would not get coalesced and would
therefore trigger a message per descendant per day (doubling or tripling the
intended spam rate).

Ordering by source_id should fix the issue as it ought make all PRs
forward-ported from the same thing contiguous, and therefore grouped
together before sending reminder messages.

An alternatively solution would be to use `groupby` instead of `search` but
it would require more modifications as we'd need to re-browse the sources
and descendants, etc...

First part of fixing #285 as this is likely why odoo/enterprise#7204 got
spammed so much: its descendants were odoo/enterprise#7367 and
odoo/enterprise#7369 and it just so happens that odoo/enterprise#7368 was
*also* a forward port PR, causing the issue explained above.

forwardport/models/project.py

@@ -971,7 +971,7 @@ stderr:
             ('state', 'not in', ['merged', 'closed']),
             # last updated more than <cutoff> ago
             ('write_date', '<', cutoff),
-        ]), lambda p: p.source_id):
+        ], order='source_id, id'), lambda p: p.source_id):
             self.env['runbot_merge.pull_requests.feedback'].create({
                 'repository': source.repository.id,
                 'pull_request': source.number,