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>

refs.h

@@ -3,8 +3,10 @@
 
 struct object_id;
 struct ref_store;
+struct repository;
 struct strbuf;
 struct string_list;
+struct string_list_item;
 struct worktree;
 
 /*