list: avoid incompatibility with *BSD sys/queue.h

The OS X build pulls in sys/queue.h, which pollutes the preprocessor
namespace with a macro generically named LIST_HEAD, and clashes with
the name we use here.

ref: http://mid.gmane.org/FB76544F-16F7-45CA-9649-FD62EE44B0DE@gmail.com

Reported-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list.h

@@ -36,6 +36,8 @@ struct list_head {
 	struct list_head *next, *prev;
 };
 
+/* avoid conflicts with BSD-only sys/queue.h */
+#undef LIST_HEAD
 /* Define a variable with the head and tail of the list. */
 #define LIST_HEAD(name) \
 	struct list_head name = { &(name), &(name) }