[FIX] *: ensure I don't get bollocked up again by tags

Today (or really a month ago) I learned: when giving git a symbolic
ref (e.g. a ref name), if it's ambiguous then

1. If `$GIT_DIR/$name` exists, that is what you mean (this is usually
   useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`,
   `REBASE_HEAD`, `REVERT_HEAD`, `CHERRY_PICK_HEAD`, `BISECT_HEAD` and
   `AUTO_MERGE`)
2. otherwise, `refs/$name` if it exists
3. otherwise, `refs/tags/$name` if it exists
4. otherwise, `refs/heads/$name` if it exists
5. otherwise, `refs/remotes/$name` if it exists
6. otherwise, `refs/remotes/$name/HEAD` if it exists

This means if a tag and a branch have the same name and only the name
is provided (not the full ref), git will select the tag, which gets
very confusing for the mergebot as it now tries to rebase onto the tag
(which because that's not fun otherwise was not even on the branch of
the same name).

Fix by providing full refs to `rev-parse` when trying to retrieve the
head of the target branches. And as a defense in depth opportunity,
also exclude tags when fetching refs by spec: apparently fetching a
specific commit does not trigger the retrieval of tags, but any sort
of spec will see the tags come along for the ride even if the tags are
not in any way under the fetched ref e.g. `refs/heads/*` will very
much retrieve the tags despite them being located at `refs/tags/*`.

Fixes #922

forwardport/models/project.py

@@ -362,7 +362,8 @@ stderr:
 {err}
 """
 
-            target_head = source.stdout().rev_parse(target_branch.name).stdout.decode().strip()
+            target_head = source.stdout().rev_parse(f"refs/heads/{target_branch.name}")\
+                .stdout.decode().strip()
             commit = conf.commit_tree(
                 tree=tree.stdout.decode().splitlines(keepends=False)[0],
                 parents=[target_head],
@@ -386,7 +387,7 @@ stderr:
         logger = _logger.getChild(str(self.id)).getChild('cherrypick')
 
         # target's head
-        head = repo.stdout().rev_parse(branch).stdout.decode().strip()
+        head = repo.stdout().rev_parse(f"refs/heads/{branch}").stdout.decode().strip()
 
         commits = self.commits()
         logger.info(

runbot_merge/models/project_freeze/__init__.py

@@ -217,7 +217,7 @@ class FreezeWizard(models.Model):
             for r in self.project_id.repo_ids
         }
         for repo, copy in repos.items():
-            copy.fetch(git.source_url(repo), '+refs/heads/*:refs/heads/*')
+            copy.fetch(git.source_url(repo), '+refs/heads/*:refs/heads/*', no_tags=True)
         all_prs = self.release_pr_ids.pr_id | self.bump_pr_ids.pr_id
         for pr in all_prs:
             repos[pr.repository].fetch(

runbot_merge/models/stagings_create.py

@@ -244,7 +244,8 @@ def staging_setup(
             # 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}',
-            *(pr.head for pr in by_repo.get(repo, []))
+            *(pr.head for pr in by_repo.get(repo, [])),
+            no_tags=True,
         )
         original_heads[repo] = head
         staging_state[repo] = StagingSlice(gh=gh, head=head, repo=source.stdout().with_config(text=True, check=False))