Pass through nit branch creation args

[sedge-bot]

Summary

• pass positional nit branch <name> <rev> forms through to git instead of routing them to native list-only branch output
• keep conformance portable in this Linux/libgit2 verification runtime by pinning the temp repo default branch, avoiding bc/xxd, and supporting util-linux script -c
• update libgit2 OID buffer constants for the available 1.5 headers

Verification

• zig build
• zig build test
• ./tests/conformance.sh /tmp/nit-wrapper-20260526.sh (144 passed / 0 failed)
• git diff --check