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/alias.c b/alias.c
index 0526304..de8e6a3 100644
--- a/alias.c
+++ b/alias.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 
 struct config_alias_data {
 	const char *alias;