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>

unpack-trees.h

@@ -1,11 +1,14 @@
 #ifndef UNPACK_TREES_H
 #define UNPACK_TREES_H
 
-#include "tree-walk.h"
+#include "cache.h"
 #include "argv-array.h"
+#include "string-list.h"
+#include "tree-walk.h"
 
 #define MAX_UNPACK_TREES 8
 
+struct cache_entry;
 struct unpack_trees_options;
 struct exclude_list;