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>

notes-utils.h

@@ -3,6 +3,9 @@
 
 #include "notes.h"
 
+struct commit_list;
+struct object_id;
+
 /*
  * Create new notes commit from the given notes tree
  *