Builds vendored libgit2 as an esp-idf component so it inherits the
lwip/mbedtls/pthread/vfs/newlib include+link graph -- the include cascade
that sank CFLAGS injection never appears. Configured for mbedTLS via a
hand-written git2_features.h. Small port surface, libgit2 sources
untouched: poll.h shim, lstat==stat (esp_port.h), p_mmap via malloc+read
(esp_map.c), and identity/symlink stubs (esp_stubs.c). Registers empty
when LIBGIT2_SRC is unset so the editor build is unaffected.
Pins the exact version libgit2-sys 0.18.5 expects, so git2's safe API can
bind it in system mode. Sits under the esp-idf component that builds it;
clone with --recurse-submodules to fetch the source.
