connect: have ref processing code take struct packet_reader

connect: have ref processing code take struct packet_reader

In a future patch, we'll want to access multiple members from struct
packet_reader when parsing references.  Therefore, have the ref parsing
code take pointers to struct reader instead of having to pass multiple
arguments to each function.

Rename the len variable to "linelen" to make it clearer what the
variable does in light of the variable change.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>