Use libgit2 OID hex size constant

[sedge-bot]

Summary

• replace the removed/unsupported libgit2 SHA1-specific OID hex-size constant with the generic GIT_OID_HEXSZ constant in log/show formatting
• keeps the fixed-size oid buffers aligned with the libgit2 1.5 headers available in the Sedge runtime

Verification

• CPATH=/opt/sedge/.local/libgit2-dev/usr/include LIBRARY_PATH=/opt/sedge/.local/libgit2/usr/lib/aarch64-linux-gnu:/opt/sedge/.local/libgit2/usr/lib zig build
• CPATH=/opt/sedge/.local/libgit2-dev/usr/include LIBRARY_PATH=/opt/sedge/.local/libgit2/usr/lib/aarch64-linux-gnu:/opt/sedge/.local/libgit2/usr/lib zig build test
• /lib/ld-linux-aarch64.so.1 --library-path /opt/sedge/.local/libgit2/usr/lib/aarch64-linux-gnu:/opt/sedge/.local/libgit2/usr/lib ./zig-out/bin/nit --help
• git diff --check github/main..HEAD