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>

config.h

@@ -1,6 +1,11 @@
 #ifndef CONFIG_H
 #define CONFIG_H
 
+#include "hashmap.h"
+#include "string-list.h"
+
+struct object_id;
+
 /* git_config_parse_key() returns these negated: */
 #define CONFIG_INVALID_KEY 1
 #define CONFIG_NO_SECTION_OR_NAME 2