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>

ll-merge.h

@@ -5,6 +5,8 @@
 #ifndef LL_MERGE_H
 #define LL_MERGE_H
 
+#include "xdiff/xdiff.h"
+
 struct ll_merge_options {
 	unsigned virtual_ancestor : 1;
 	unsigned variant : 2;	/* favor ours, favor theirs, or union merge */