fix: fetching and cloning with refspecs that are tags (in shallow clones)

[Sebastian Thiel (Byron)]

Fixes #2554.

Tasks

• refackiew

---

Summary

Fix shallow clones with an explicit tag ref by resolving the requested ref name against the remote before constructing the shallow single-ref refspec.

This addresses #2554, where PrepareFetch::with_shallow(...).with_ref_name(Some("tag")) incorrectly treated the requested name as a local branch and generated a refspec like:

+refs/heads/<tag>:refs/remotes/origin/<tag>

That refspec does not match tag-only refs and now fails once fetch refspec validation rejects unmatched required mappings.

Research

I checked baseline Git behavior using the local Git checkout at:

/Users/byron/dev/github.com/git/git

For a non-shallow clone with --branch <tag>, Git keeps the normal wildcard fetch refspec:

+refs/heads/*:refs/remotes/origin/*

For a shallow clone with --depth 1 --branch <tag>, Git writes a tag-to-tag refspec instead:

+refs/tags/<tag>:refs/tags/<tag>

The fix follows that behavior: shallow tag clones fetch the selected tag into refs/tags/*, while shallow branch clones continue to fetch into refs/remotes/<remote>/*.

Changes

• Resolve with_ref_name() against the remote refs when using the shallow single-ref clone optimization.
• Use the resolved full ref name when constructing the shallow refspec.
• Preserve branch behavior by mapping refs/heads/<name> to refs/remotes/<remote>/<name>.
• Map non-branch refs, including tags, to themselves.
• Extend the annotated tag clone test to cover both non-shallow and shallow clones in one test.
• Assert that the shallow tag clone stores a tag refspec of the form:

+refs/tags/<tag>:refs/tags/<tag>