Force fetch tags on checkout local working clone

[hiddeco]

In Git 2.20 "git fetch" was taught to forbid updates to existing tags
without the "--force" option1. As Flux force-pushes a tag all the
time to keep track of the sync state, this would sometimes lead to
'would clobber existing tag' errors while making sure a fresh checkout
was up-to-date with its mirror, as described issue 21692.

To prevent this from happening: force fetch all tags from the mirror
while creating the local working clone, and before fetching anything
else.

NB: We only have to do this while creating a working clone from a
mirror, as a mirror (bare clone) will accept non-fast-forward tag
changes from remote.

Fixes #2169

[squaremo]

This makes me nervous because it will apply to all refs fetched from the origin, not just the tag. That makes me think perhaps we should pull the specific tag, with a +, separately to everything else.

[hiddeco]

This makes me nervous because it will apply to all refs fetched from the origin, not just the tag.

This is already the case if I am reading the man page correctly.

Unlike when pushing with git-push(1), any updates
outside of refs/{tags,heads}/* will be accepted
without + in the refspec (or --force), whether
that’s swapping e.g. a tree object for a blob, or a
commit for another commit that’s doesn’t have the
previous commit as an ancestor etc.

[squaremo]

any updates outside of refs/{tags,heads}/* will be accepted
without + in the refspec

Since refs/heads/* are branches, that statement does not include branches; i.e., git will only fast-forward branches, when fetching.

[hiddeco]

@squaremo this should be it.

[squaremo]

It's tricky to reproduce the original problem, but I think this does on paper address it.
It also behaves the same as before otherwise, as far as I can tell from actually trying it. So 👍