Configure repos to ignore tags, and force branches when rev-parsing

[xmo-odoo]

odoo/o-spreadsheet#4722 got forward-ported to 17.0 as 66 commits instead of the one (odoo/o-spreadsheet#4740).

edit: also odoo/o-spreadsheet#4749 so it happened twice on spreadsheet porting from 16 to 17 and a quick check didn't reveal anything in odoo/odoo or odoo/enterprise, so might be something in o-spreadsheet which confuses the WC-less runbot forward porting thingie

edit 2: also odoo/o-spreadsheet#4815 which doesn't have conflicts so that's not where the issue is.

As it turns out the issue is that o-spreadsheet uses a shit-ton of tags, and they fucked up the 17.0.0 tag: it was created as 17.0, then deleted. Except the mergebot managed to fetch it before that and git does not prune tags by default.

And when a ref is ambiguous rev-parse favors tags

When ambiguous, a is disambiguated by taking the first match in the following rules:

1. If $GIT_DIR/ 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/ if it exists;
3. otherwise, refs/tags/ if it exists;
4. otherwise, refs/heads/ if it exists;
5. otherwise, refs/remotes/ if it exists;
6. otherwise, refs/remotes//HEAD if it exists.

So git would resolve 17.0 to refs/tags/17.0, it would complain that "refname '17.0' is ambiguous." but that pretty much got ignored (I couldn't even find the info in journalctl).

The information was in the logs but I didn't notice it:

runbot_merge.pull_requests(234982,): copy 1 commits to 17.0 (5d60210a896b51c244af46c03978a62f7cf892cd)

which is the commit for the tag 17.0, completely different of and unrelated to the branch of the same name. As such the mergebot would dutifully rebase the commits from the source PR onto that tag, and create the conflict commits over there too (for the same reasons).

---

• git fetch: By default, any tag that points into the histories being fetched is also fetched; the effect is to fetch tags that point at branches that you are interested in. This default behavior can be changed by using the --no-tags options or by configuring remote.<name>.tagOpt (so remote.origin.tagOpt = --no-tags)
• git rev-parse: force refs/heads/ prefix branches

Ancillary notes from a bit of testing:

• fetching an exact commit does not seem to retrieve tags (?)
• fetching a ref by full refspec does fetch tags regardless