Fix header breakage due to redefining PATH_MAX.

The header builtin.h was, incorrectly, redefining PATH_MAX which
causes a header order dependency in builtin-write-tree.c.  The fix
is to simply include <limits.h> directly to obtain the correct
definition of PATH_MAX.
Signed-off-by: NRamsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin.h

@@ -2,10 +2,7 @@
 #define BUILTIN_H
 
 #include <stdio.h>
-
-#ifndef PATH_MAX
-# define PATH_MAX 4096
-#endif
+#include <limits.h>
 
 extern const char git_version_string[];