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/ident.c b/ident.c
index bea871c..d41fc91 100644
--- a/ident.c
+++ b/ident.c
@@ -6,6 +6,7 @@
  * Copyright (C) 2005 Linus Torvalds
  */
 #include "cache.h"
+#include "config.h"
 
 static struct strbuf git_default_name = STRBUF_INIT;
 static struct strbuf git_default_email = STRBUF_INIT;