do not include the same header twice

A few files include the same header file directly more than once.

As all these headers protect themselves against repeated inclusion
by the "#ifndef FOO_H / #define FOO_H / ... / #endif" idiom, leave
only the first inclusion and remove the later inclusion as a no-op
clean-up.

Signed-off-by: Дилян Палаузов <git-dpa@aegee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

userdiff.c

@@ -1,6 +1,5 @@
 #include "cache.h"
 #include "userdiff.h"
-#include "cache.h"
 #include "attr.h"
 
 static struct userdiff_driver *drivers;