teach get_remote_heads to read from a memory buffer

teach get_remote_heads to read from a memory buffer

Now that we can read packet data from memory as easily as a
descriptor, get_remote_heads can take either one as a
source. This will allow further refactoring in remote-curl.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>