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>

connected.h

@@ -1,6 +1,7 @@
 #ifndef CONNECTED_H
 #define CONNECTED_H
 
+struct object_id;
 struct transport;
 
 /*