test: add git edge-case test matrix

[rocketman-code]

Context

Parent: #160

15 git tests existed pre-v0.4.0 but missed annotated tags (#145) and branch names with slashes (#146). Tests covered the common cases (lightweight tags, simple branch names, SHAs) but not the edges that real-world git repos produce.

Expected Behavior

Systematic coverage of classify_diff_arg() and GitTreeVfs against a matrix of git ref types.

Scope

• Annotated tags (git tag -a)
• Lightweight tags
• Branch names with slashes (feature/auth, fix/bug/123)
• Detached HEAD
• Short SHAs (7-char, full 40-char)
• HEAD~N, HEAD^N syntax
• Tags pointing to tags (peeled chain)
• Ambiguous refs (file named "main" + branch named "main")
• Empty repo (no commits)
• Shallow clones (--depth 1)
• GitTreeVfs reading files from various ref types