[FIX] runbot: build PR with corresponding community PR

When searching for a matching PR in a target repo, the match was made on
the 'base ref' which is the base branch that the PR targets.
This means that the second case of the _get_closest_branch method was
never reached.

Example:
    An enterprise PR is created that targets Odoo branch 12.0 but with a
    matching community PR with a branch with the same name.
    The corresponding PR is never found by the runbot because the first
    rule match: '12.0' is the base ref of the PR.

This could have been fixed by using the branch pull_head_name field to
build the domain but that leads to a problem with the second rule:

If a PR branch is named 'patch-1', the domain will match each PR with
the same pull_head_name.

With this commit, the pull_head_name field will store the pull head
label. It's not a problem with older PR as a newly created PR in
enterprise will not accidentally match with older ones.

Another issue appeared with github branch naming like 'patch-'.
If someone creates a PR with a branch auto-named 'patch-1',
targeting the community repo and later creates an unrelated PR,
targetting another repo depending of the community, they will be
matched.

To avoid that, the pull head names that ends with 'patch-n' are not
stored. Those pull requests will be built against the target branch
head.

The actual behavior, before this commit, is a blocking point for the
runbot_merge which is based on PR's only. It means that when a community
PR is wrongly matched with with an enterprise PR runbot_merge will not
merge the PR's.

runbot/models/branch.py

@@ -6,7 +6,7 @@ from odoo import models, fields, api
 
 _logger = logging.getLogger(__name__)
 _re_coverage = re.compile(r'\bcoverage\b')
-
+_re_patch = re.compile(r'.*patch-\d+$')
 
 class runbot_branch(models.Model):
 
@@ -72,8 +72,9 @@ class runbot_branch(models.Model):
         """compute pull head name"""
         for branch in self:
             pi = self._get_pull_info()
-            if pi:
-                branch.pull_head_name = pi['head']['ref']
+            if pi and not _re_patch.match(pi['head']['label']):
+                # label is used to disambiguate PR with same branch name
+                branch.pull_head_name = pi['head']['label']
 
     def _get_branch_quickconnect_url(self, fqdn, dest):
         self.ensure_one()

runbot/models/build.py

@@ -133,8 +133,7 @@ class runbot_build(models.Model):
 
         build = self
         branch, repo = build.branch_id, build.repo_id
-        pi = branch._get_pull_info()
-        name = pi['base']['ref'] if pi else branch.branch_name
+        name = branch.pull_head_name or branch.branch_name
 
         target_repo = self.env['runbot.repo'].browse(target_repo_id)