fix: Allow any ref of a Git repo in the fragment

[Leonidas-from-XIV]

This reverses the split of branches and tags from #11127, instead putting all possible references (e.g. also pull requests) in the same map and implements short-hands for looking up unqualified refs.

Fixes #11140

[rgrinberg]

Tbh, this doesn't seem like an improvement in readability. I think you could just as easily just parse the refs/{heads,tags} out of the query and then know which map you need to look in.

Opinions aside, I think this makes it a bit harder to implement the "did you mean" hint. That one would be rather useful when a user typos a branch or a tag. IMO you should hold off this refactoring until we have that useful hint ready.

[Leonidas-from-XIV]

The problem is with other types of refs like pull refs for heads and merges. The dune repo itself has at least 4 types of refs and we don't even do anything particularly unusual with Git.

Git refs are basically just a Object.resolved String.Map.t and the string is just refs/{heads,tags} by convention, so to me this seems like the best way to represent them aligned with how Git treats them, thus fewer concepts to keep in mind and less ways for repos to break our assumptions.

[rgrinberg]

Approving as this PR fixes the issue reliably like it advertises. Not a fan of keeping map where everything is prefixed with refs/{heads,tags} - I would prefer the types to reflect the structure, and help improve the hints.

[Leonidas-from-XIV]

Not a fan of keeping map where everything is prefixed with refs/{heads,tags}

I understand, however the way it works in opam is that every ref in ls-remote can be used. In particular that means that it can use refs like refs/pull/9306/head and refs/pull/9306/merge (these are taken from git ls-remote [email protected]:ocaml/dune.git), whereas if we create a map for each type this becomes an ongoing maintenance process of adding more and more types of references as github/git/users add more references.

We could of course also say that we only support branch and tag references because it's unlikely someone would use them, however that feels like an easy feature to support at the cost of a slightly messy map.