remote-curl: tighten "version 2" check for smart-http

remote-curl: tighten "version 2" check for smart-http

In a v2 smart-http conversation, the server should reply to our initial
request with a pkt-line saying "version 2". We check that with
starts_with(), but really that should be the only thing in that packet.
A response of "version 20" should not match.

Let's tighten this check to use strcmp(). Note that we don't need to
worry about a trailing newline here, because the ptk-line code will have
chomped it for us already.

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