Make fetch a builtin

Thanks to Johannes Schindelin for review and fixes, and Julian
Phillips for the original C translation.

This changes a few small bits of behavior:

branch.<name>.merge is parsed as if it were the lhs of a fetch
refspec, and does not have to exactly match the actual lhs of a
refspec, so long as it is a valid abbreviation for the same ref.

branch.<name>.merge is no longer ignored if the remote is configured
with a branches/* file. Neither behavior is useful, because there can
only be one ref that gets fetched, but this is more consistant.

Also, fetch prints different information to standard out.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -492,6 +492,7 @@ struct ref {
 	unsigned char old_sha1[20];
 	unsigned char new_sha1[20];
 	unsigned char force;
+	unsigned char merge;
 	struct ref *peer_ref; /* when renaming */
 	char name[FLEX_ARRAY]; /* more */
 };