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>

attr.h

@@ -7,6 +7,7 @@ struct git_attr;
 /* opaque structures used internally for attribute collection */
 struct all_attrs_item;
 struct attr_stack;
+struct index_state;
 
 /*
  * Given a string, return the gitattribute object that