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>
diff --git a/builtin/am.c b/builtin/am.c
index a95dd8b..59f46a2 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -4,6 +4,7 @@
  * Based on git-am.sh by Junio C Hamano.
  */
 #include "cache.h"
+#include "config.h"
 #include "builtin.h"
 #include "exec_cmd.h"
 #include "parse-options.h"