[FIX] forwardport: completely update PR data when forwarding updates

The process did properly update the state, but not the squash state.

It's somewhat unclear whether the state should be fully reset and
require reapproval though. Maybe only the validation should be reset?
The CI will eventually run and either succeed (re-validating) or
fail (devalidating, hopefully) but I'm not entirely sure this is
correct.

forwardport/models/forwardport.py

@@ -117,8 +117,16 @@ class UpdateQueue(models.Model, Queue):
                     child.target, child.refname, s)
 
                 new_head = working_copy.stdout().rev_parse(child.refname).stdout.decode().strip()
+                commits_count = int(working_copy.stdout().rev_list(
+                    f'{child.target.name}..{child.refname}',
+                    count=True
+                ).stdout.decode().strip())
                 # update child's head to the head we're going to push
-                child.with_context(ignore_head_update=True).head = new_head
+                child.with_context(ignore_head_update=True).write({
+                    'head': new_head,
+                    # 'state': 'opened',
+                    'squash': commits_count == 1,
+                })
                 working_copy.push('-f', 'target', child.refname)
                 # committing here means github could technically trigger its
                 # webhook before sending a response, but committing before