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>

url.h

@@ -1,6 +1,8 @@
 #ifndef URL_H
 #define URL_H
 
+struct strbuf;
+
 extern int is_url(const char *url);
 extern int is_urlschemechar(int first_flag, int ch);
 extern char *url_decode(const char *url);