Correct fscanf formatting string for I64u values

This fix is probably purely cosmetic because PRIuMAX is likely identical
to SCNuMAX. Nevertheless, when using a function of the scanf() family,
the correct interpolation to use is the latter, not the former.

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

git-compat-util.h

@@ -237,6 +237,10 @@ extern char *gitbasename(char *);
 #define PRIuMAX "llu"
 #endif
 
+#ifndef SCNuMAX
+#define SCNuMAX PRIuMAX
+#endif
+
 #ifndef PRIu32
 #define PRIu32 "u"
 #endif