mingw: use strftime() directly in UCRT builds

The `mingw_strftime()` wrapper exists to work around msvcrt.dll's
incomplete `strftime()` implementation by dynamically loading the
version from ucrtbase.dll at runtime via `LoadLibrary()` +
`GetProcAddress()`. When the binary is already linked against UCRT
(i.e. when building in the UCRT64 environment), the linked-in
`strftime()` is the ucrtbase.dll version, making the dynamic loading
needless churn: It's calling the very same code.

Simply guard both the declaration and implementation so that the
unnecessary work-around is skipped in UCRT builds.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1394,6 +1394,9 @@ revert_attrs:
 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;
@@ -1404,6 +1407,7 @@ 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);