connect: convert get_remote_heads to use struct packet_reader
In order to allow for better control flow when protocol_v2 is introduced
convert 'get_remote_heads()' to use 'struct packet_reader' to read
packet lines. This enables a client to be able to peek the first line
of a server's response (without consuming it) in order to determine the
protocol version its speaking and then passing control to the
appropriate handler.
This is needed because the initial response from a server speaking
protocol_v0 includes the first ref, while subsequent protocol versions
respond with a version line. We want to be able to read this first line
without consuming the first ref sent in the protocol_v0 case so that the
protocol version the server is speaking can be determined outside of
'get_remote_heads()' in a future patch.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>