mingw: use strftime() directly in UCRT builds (#6130)

Currently, Git for Windows is built off of the MINGW64 tool chain. But
this will have to change because [the MSYS2 project deprecated this tool
chain in favor of
UCRT64](https://www.msys2.org/news/#2026-03-15-deprecating-the-mingw64-environment).
Of course, that's only possible because they dropped support for Windows
8.1, which Git for Windows will probably have to do relatively soon. The
best time to do that is probably [the Git 3.0 inflection
point](#6018) when we
already promised to drop support for older Windows versions.

To prepare for such a huge change, I investigated what needs to be
changed in Git for Windows' source code. And the good news is there's
actually not very much. This here patch seems to be the only change
that's necessary, and not even _strictly_ necessary: the
`mingw_strftime()` wrapper would still do the right thing. It would just
uselessly load the same function that's already loaded, dynamically,
again.

- The `strerror()` override [is guarded by an `#ifndef
_UCRT`](https://github.com/git-for-windows/git/blob/v2.53.0.windows.2/compat/mingw-posix.h#L294-L296),
- `PRIuMAX` resolves to standard `"llu"` [via
`<inttypes.h>`](https://github.com/git-for-windows/git/blob/v2.53.0.windows.2/compat/mingw-posix.h#L449-L454)
(note that `__MINGW64_VERSION_MAJOR` is defined both in MINGW64 and
UCRT64, by virtue of using the `mingw-w64-headers`),
-
[`__USE_MINGW_ANSI_STDIO=0`](https://github.com/git-for-windows/git/blob/v2.53.0.windows.2/config.mak.uname#L751C19-L751C33)
is irrelevant because [`_UCRT` short-circuits
it](https://github.com/git-for-windows/git-sdk-64/blob/08933e673c79b5db48419917a2b02746b390afc4/mingw64/include/inttypes.h#L33),
and
- `SNPRINTF_RETURNS_BOGUS` hasn't been set for Git for Windows' builds
since ec47a33, i.e. for a _really_ long
time.

Author: dscho

compat/mingw.c

@@ -1509,6 +1509,9 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 size_t mingw_strftime(char *s, size_t max,
 		      const char *format, const struct tm *tm)
 {
+#ifdef _UCRT
+	size_t ret = strftime(s, max, format, tm);
+#else
 	/* a pointer to the original strftime in case we can't find the UCRT version */
 	static size_t (*fallback)(char *, size_t, const char *, const struct tm *) = strftime;
 	size_t ret;
@@ -1519,6 +1522,7 @@ size_t mingw_strftime(char *s, size_t max,
 		ret = strftime(s, max, format, tm);
 	else
 		ret = fallback(s, max, format, tm);
+#endif
 
 	if (!ret && errno == EINVAL)
 		die("invalid strftime format: '%s'", format);