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

[sedge-bot]

Heartbeat verification/update for PR #3:

• Found the local Linux conformance harness was failing before the PR code could be trusted in this runtime because it assumed a default main branch, used unavailable bc/xxd, and used BSD-style script arguments.
• Pushed f4a8a0b to sedge/libgit2-oid-hexsz to make the harness portable: git init -q -b main, awk/od helpers, and a Linux/BSD script_tty wrapper.

Local verification in the Sedge Linux/OrbStack runtime:

• zig build passed with Zig 0.15.2 and local libgit2 1.5 headers/libs.
• zig build test passed.
• bash -n tests/conformance.sh passed.
• ./tests/conformance.sh /tmp/nit-pr3-wrapper.sh passed: 143 passed / 0 failed.
• git diff --check github/main..HEAD passed.

Runtime note: the wrapper invokes /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 so the harness exercises the built binary against the local libgit2 runtime without exporting LD_LIBRARY_PATH.