Cleanup duplicate initialization code in transport_get

Cleanup duplicate initialization code in transport_get

We always allocate and return a struct transport* right now as every
URL is considered to be a native Git transport if it is not rsync,
http/https/ftp or a bundle.  So we can simplify the initialization
of a new transport object by performing one xcalloc call and filling
in only the attributes required.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>