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/merge-recursive.c b/merge-recursive.c
index 62decd5..a85144c 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -4,6 +4,7 @@
  * The thieves were Alex Riesen and Johannes Schindelin, in June/July 2006
  */
 #include "cache.h"
+#include "config.h"
 #include "advice.h"
 #include "lockfile.h"
 #include "cache-tree.h"