http: factor out matching of curl http/2 trace lines

http: factor out matching of curl http/2 trace lines

We have to parse out curl's http/2 trace lines so we can redact their
headers. We already match two different types of lines from various
vintages of curl. In preparation for adding another (which will be
slightly more complex), let's pull the matching into its own function,
rather than doing it in the middle of a conditional.

While we're doing so, let's expand the comment a bit to describe the two
matches. That probably should have been part of db30130165 (http: handle
both "h2" and "h2h3" in curl info lines, 2023-06-17), but will become
even more important as we add new types.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>