mingw: avoid mktemp() in mkstemp() implementation

The implementation of mkstemp() for MinGW uses mktemp() and open()
without the flag O_EXCL, which is racy.  It's not a security problem
for now because all of its callers only create files within the
repository (incl. worktrees).  Replace it with a call to our more
secure internal function, git_mkstemp_mode(), to prevent possible
future issues.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1044,10 +1044,7 @@ char *mingw_mktemp(char *template)
 
 int mkstemp(char *template)
 {
-	char *filename = mktemp(template);
-	if (filename == NULL)
-		return -1;
-	return open(filename, O_RDWR | O_CREAT, 0600);
+	return git_mkstemp_mode(template, 0600);
 }
 
 int gettimeofday(struct timeval *tv, void *tz)