compat/msvc: use _chsize_s for ftruncate

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>