[IMP] forwardport: add logging to branch reinsertion

When a new branch is created between other branches, the process to
try and look for forward-port PRs to create to "fill in" currently has
no logging so it's very difficult to figure out why it decides not to
do something.

Add some logging to the process to try and better understand what
happens.

Fixes #441

forwardport/models/project.py

@@ -99,6 +99,9 @@ class Project(models.Model):
             bafter = p._forward_port_ordered()
             if bafter.ids == bbefore.ids:
                 continue
+
+            logger = _logger.getChild('project').getChild(p.name)
+            logger.debug("branches updated %s -> %s", bbefore, bafter)
             # if it's just that a branch was inserted at the end forwardport
             # should keep on keeping normally
             if bafter.ids[:-1] == bbefore.ids:
@@ -120,6 +123,7 @@ class Project(models.Model):
                     if new:
                         raise UserError("Inserting multiple branches at the same time is not supported")
                     new = b
+            logger.debug('before: %s new: %s after: %s', before.ids, new.ids, after.ids)
             # find all FPs whose ancestry spans the insertion
             leaves = self.env['runbot_merge.pull_requests'].search([
                 ('state', 'not in', ['closed', 'merged']),
@@ -133,6 +137,7 @@ class Project(models.Model):
                 '|', ('id', 'in', leaves.mapped('source_id').ids),
                      ('source_id', 'in', leaves.mapped('source_id').ids),
             ])
+            logger.debug("\nPRs spanning new: %s\nto port: %s", leaves, candidates)
             # enqueue the creation of a new forward-port based on our candidates
             # but it should only create a single step and needs to stitch batch
             # the parents linked list, so it has a special type