compat/msvc: use _chsize_s for ftruncate

On Windows, `unsigned long` and `long` are 32 bits even on 64-bit builds. The MSVC compatibility header has shimmed `ftruncate()` with #define ftruncate _chsize ever since `compat/msvc-posix.h` was introduced. `_chsize()` takes a 32-bit `long` for the new length, which silently truncates files (and the requested size) to 2 GiB. That is enough to make t7508 test 126 "git add fails gracefully with 4 GiB and 8 GiB files" fail under MSVC: `test-tool truncate` creates a sparse 4 GiB or 8 GiB file via the shimmed `ftruncate()`, and the test never gets off the ground. `_chsize_s()` is the modern replacement, accepts a 64-bit `__int64` length, and is the only sensible target on Windows. The catch is that it does not follow the POSIX `-1` + `errno` convention: it returns `0` on success and an errno value (a small positive integer) on failure. A plain `#define ftruncate _chsize_s` would therefore silently break callers that test the return value as `< 0` or against `-1`, of which there are several: `http.c`, `parallel-checkout.c`, and `t/helper/test-truncate.c` among them. Introduce a `static inline` wrapper that calls `_chsize_s()`, copies its errno return into `errno`, and translates the result to the familiar `-1` / `0` convention, then point `ftruncate` at the wrapper. Place the wrapper after `#include "mingw-posix.h"` so the `off_t` parameter resolves to the already-widened `off64_t` rather than the 32-bit `_off_t` from `compat/vcbuild/include/unistd.h`. MinGW is unaffected: its `ftruncate()` already takes `off_t` and routes through `ftruncate64()` when `_FILE_OFFSET_BITS=64`, which is the default in our build. Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>