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/pretty.c b/pretty.c
index d0f86f5..06a1f13 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 #include "commit.h"
 #include "utf8.h"
 #include "diff.h"