remote-curl: refactor reading into rpc_state's buf

remote-curl: refactor reading into rpc_state's buf

Currently, whenever remote-curl reads pkt-lines from its response file
descriptor, only the payload is written to its buf, not the 4 characters
denoting the length. A future patch will require the ability to also
write those 4 characters, so in preparation for that, refactor this read
into its own function.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>