Add missing includes and forward declarations

I looped over the toplevel header files, creating a temporary two-line C
program for each consisting of
  #include "git-compat-util.h"
  #include $HEADER
This patch is the result of manually fixing errors in compiling those
tiny programs.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -4,6 +4,7 @@
 #include "git-compat-util.h"
 #include "repository.h"
 #include "string-list.h"
+#include "cache.h"
 
 struct commit;