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.h

@@ -3,6 +3,9 @@
 
 #include "string-list.h"
 
+struct object_id;
+struct strbuf;
+
 /*
  * Function type for combining two notes annotating the same object.
  *