[IMP] *: avoid updating (all) branches in cases where that's unnecessary

Partial mitigation of #1065, not complete by any means. Avoiding
updating a local ref during staging is probably the most important bit
here, we're staging on commits (into a new ref) anyway so it should
not be needed, and that avoids conflicts between the staging cron and
e.g. the forwardport cron.

forwardport/models/project.py

@@ -331,11 +331,14 @@ class PullRequests(models.Model):
             "Forward-porting" if forward else "Back-porting",
             self.display_name, root.display_name, target_branch.name
         )
-        fetch = source.with_config(stdout=subprocess.PIPE, stderr=subprocess.STDOUT).fetch()
-        logger.info("Updated cache repo %s:\n%s", source._directory, fetch.stdout.decode())
 
         head_fetch = source.with_config(stdout=subprocess.PIPE, stderr=subprocess.STDOUT, check=False) \
-            .fetch(git.source_url(self.repository), root.head)
+            .fetch(
+                git.source_url(self.repository),
+                f"refs/heads/{target_branch.name}:refs/heads/{target_branch.name}",
+                root.head,
+                no_tags=True,
+            )
         if head_fetch.returncode:
             raise ForwardPortError(
                 f"During forward port of {self.display_name}, unable to find "

runbot_merge/models/patcher.py

@@ -263,7 +263,7 @@ class Patch(models.Model):
                 remote,
                 f"+refs/heads/{target.name}:refs/heads/{target.name}",
                 *(p.commit for p in ps),
-                no_tags=True
+                no_tags=True,
             ).returncode:
                 r.fetch(remote, f"+refs/heads/{target.name}:refs/heads/{target.name}", no_tags=True)
                 for p in ps:

runbot_merge/models/stagings_create.py

@@ -249,12 +249,7 @@ def staging_setup(
         source = git.get_local(repo)
         source.fetch(
             git.source_url(repo),
-            # a full refspec is necessary to ensure we actually fetch the ref
-            # (not just the commit it points to) and update it.
-            # `git fetch $remote $branch` seems to work locally, but it might
-            # be hooked only to "proper" remote-tracking branches
-            # (in `refs/remotes`), it doesn't seem to work here
-            f'+refs/heads/{target.name}:refs/heads/{target.name}',
+            head,
             *(pr.head for pr in by_repo.get(repo, [])),
             no_tags=True,
         )