connect: tighten check for unexpected early hang up

connect: tighten check for unexpected early hang up

A server hanging up immediately to mark access being denied does not
send any .have refs, shallow lines, or anything else before hanging
up.  If the server has sent anything, then the hangup is unexpected.

That is, if the server hangs up after a shallow line but before sending
any refs, then git should tell me so:

	fatal: The remote end hung up upon initial contact

instead of suggesting an access control problem:

	fatal: Could not read from remote repository.
	Please make sure you have the correct access rights
	and the repository exists.

Noticed while examining this code.  This case isn't likely to come up
in practice but tightening the check makes the code easier to read and
manipulate.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>