remote-curl: handle URLs without protocol

Generally remote-curl would never see a URL that did not
have "proto:" at the beginning, as that is what tells git to
run the "git-remote-proto" helper (and git-remote-http, etc,
are aliases for git-remote-curl).

However, the special syntax "proto::something" will run
git-remote-proto with only "something" as the URL. So a
malformed URL like:

  http::/example.com/repo.git

will feed the URL "/example.com/repo.git" to
git-remote-http. The resulting URL has no protocol, but the
code added by 372370f (http: use credential API to handle
proxy authentication, 2016-01-26) does not handle this case
and segfaults.

For the purposes of this code, we don't really care what the
exact protocol; only whether or not it is https. So let's
just assume that a missing protocol is not, and curl will
handle the real error (which is that the URL is nonsense).

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

t/t5550-http-fetch-dumb.sh

@@ -250,5 +250,13 @@ test_expect_success 'git client does not send an empty Accept-Language' '
 	! grep "^Accept-Language:" stderr
 '
 
+test_expect_success 'remote-http complains cleanly about malformed urls' '
+	# do not actually issue "list" or other commands, as we do not
+	# want to rely on what curl would actually do with such a broken
+	# URL. This is just about making sure we do not segfault during
+	# initialization.
+	test_must_fail git remote-http http::/example.com/repo.git
+'
+
 stop_httpd
 test_done