config: don't include config.h by default

Stop including config.h by default in cache.h.  Instead only include
config.h in those files which require use of the config system.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -6,6 +6,7 @@
  */
 
 #include "cache.h"
+#include "config.h"
 #include "lockfile.h"
 #include "cache-tree.h"
 #include "color.h"