compat: use git_mkdtemp()

A file might appear at the path returned by mktemp(3) before we call
mkdir(2).  Use the more robust git_mkdtemp() instead, which retries a
number of times and doesn't need to call lstat(2).

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

compat/mkdtemp.c

@@ -2,7 +2,5 @@
 
 char *gitmkdtemp(char *template)
 {
-	if (!*mktemp(template) || mkdir(template, 0700))
-		return NULL;
-	return template;
+	return git_mkdtemp(template);
 }