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>

resolve-undo.h

@@ -1,6 +1,8 @@
 #ifndef RESOLVE_UNDO_H
 #define RESOLVE_UNDO_H
 
+#include "cache.h"
+
 struct resolve_undo_info {
 	unsigned int mode[3];
 	struct object_id oid[3];