GeezerVison

Fixes #3218.

checkout_branch currently treats every selection as a remote branch name, so a tag or an explicit refs/heads/... reference cannot be selected when cloning.

This keeps branch-first lookup for short names and falls back to a tag only when the branch is absent. Explicit refs/heads/... selects only a branch; explicit refs/tags/... selects only a tag. Tag selections use the existing HEAD setter to peel annotated tags and detach HEAD at the target commit. Normal branch tracking and the remote's default HEAD are preserved.

The API documentation now describes these choices. Twelve local regression tests cover lightweight and nested annotated tags, branch/tag collisions, explicit references, bare clones, the non-optimized file transport, missing selections and non-commit tags. They also check tracking configuration, detached HEAD and cleanup on failure. Arbitrary revision expressions are not added.

Validation on Linux, based on 42a89d7cc9374a19e72fa57011e5fe3a99c7b3bf:

• Debug and Release builds with ENABLE_WERROR=ON pass. HTTPS, SSH, NTLM and Negotiate were disabled because their development dependencies are unavailable here.
• All 12 new tests pass in Debug, Release and ASan+UBSan with leak detection. With the original clone.c, seven positive regression cases fail and five control cases pass.
• Thirty local clone tests, run individually in each of Debug and Release, pass. bad_urls was not included because its network attempts timed out.
• The remaining offline Release selection (-xonline -xclone::nonetwork) reports 3,103 executions, 45 skips and no failures. The same Debug selection has one structinit padding failure, reproduced after restoring the original clone.c.

No shallow HTTP/SSH, Windows or macOS validation is claimed.