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>

log-tree.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 #include "diff.h"
 #include "commit.h"
 #include "tag.h"