The loose ODB silently dropped every write on device: utimes() returned 0
unconditionally, so libgit2's freshen probe (git_futils_touch -> p_utimes)
always reported "object exists" and git_odb_write skipped the write entirely
-- blobs/trees/commits never hit disk and write_tree failed with "invalid
object specified". Gate utimes on file existence (present -> 0, absent ->
ENOENT). Also add a remove-then-rename p_rename (FATFS f_rename can't replace
an existing target, no hardlinks), with posix.c's original scoped out in the
component CMakeLists, plus symlink/gai_strerror link stubs git_push pulls in.
p_rename was verified on hardware (cleared the rename error); the utimes fix
is diagnosed from the failure + libgit2 source and build-verified, on-device
confirmation still pending.
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.
