libgit2: fix broken build

[tc-agent]

The OSS-Fuzz build for libgit2 has been failing since 2026-05-08 (see the
build status index
for last_successful_build.finish_time). The Fuzz Introspector profile
reflects this — no recent fuzzer or coverage build is available
(profile).
The last successful coverage build (2026-05-08) reports 21.86% line coverage
(report).

Two latent problems in build.sh's hand-rolled compile invocation needed
addressing:

1. -I./include/git2 was on the compiler include path. libgit2 ships a
   Visual Studio compatibility shim at
   include/git2/stdint.h
   (a no-op outside _MSC_VER); with that directory on the include path
   the system <inttypes.h> resolves its #include <stdint.h> to the
   shim instead of clang's, leaving uintmax_t / uint64_t / uint32_t
   undefined and failing the fuzzer-side compile with "unknown type name
   'uintmax_t'". Removing the flag restores the system header search.

2. libgit2's CMake configures git2_features.h (and experimental.h) into
   \${PROJECT_BINARY_DIR}/gen_headers/ — see
   src/util/CMakeLists.txt
   and
   src/libgit2/CMakeLists.txt;
   the in-tree libgit2 build references them via LIBGIT2_INCLUDES, but
   build.sh did not have that directory on its own include path. Adding
   -I./gen_headers makes the configured headers visible to the fuzzer
   compile step.

After the fix, fuzzer compilation completes cleanly, all eight existing
fuzz targets build, and the coverage build returns to the pre-break level
documented in the 2026-05-08 report linked above (21.86% lines).

[github-actions]

Fuzzing Coverage Report

Tested: project libgit2 · base 2de60f9 → head 9573093 · 300s total fuzz budget · updated 2026-05-27 20:58 UTC · workflow run

Metric	Before	After	Delta
Static reachability	8.9% (1012/11333)	22.8% (1002/4396)	+13.9%
Line coverage	0%	19.0% (16816/88283)	new
Branch coverage	0%	16.6% (8863/53382)	new
Function coverage	0%	23.1% (989/4284)	new

Per-harness

Harness	Lines before	Lines after	Δ
commit_graph_fuzzer	0%	1.8% (1575/86806)	new
config_file_fuzzer	0%	1.9% (1688/87078)	new
download_refs_fuzzer	0%	9.1% (7946/86856)	new
midx_fuzzer	0%	1.8% (1550/86802)	new
objects_fuzzer	0%	2.4% (2107/86780)	new
packfile_fuzzer	0%	4.2% (3651/86962)	new
patch_parse_fuzzer	0%	1.9% (1630/87532)	new
revparse_fuzzer	0%	12.4% (10745/86780)	new

_{Δ = (after − before) / before, to accommodate that denominators may change. "new" when before is 0; "deleted" when after is 0.}