remote-ext: simplify git pkt-line generation

We format a pkt-line into a heap buffer, which requires
manual computation of the required size, and uses some bare
sprintf calls. We could use a strbuf instead, which would
take care of the computation for us. But it's even easier
still to use packet_write(). Besides handling the formatting
and writing for us, it fixes two things:

  1. Our manual max-size check used 0xFFFF, while technically
     LARGE_PACKET_MAX is slightly smaller than this.

  2. Our packet will now be output as part of
     GIT_TRACE_PACKET debugging.

Unfortunately packet_write() does not let us build up the
buffer progressively, so we do have to repeat ourselves a
little depending on the "vhost" setting, but the end result
is still far more readable than the original.

Since there were no tests covering this feature at all,
we'll add a few into t5802.

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

t/t5802-connect-helper.sh

@@ -69,4 +69,32 @@ test_expect_success 'update backfilled tag without primary transfer' '
 	test_cmp expect actual
 '
 
+
+test_expect_success 'set up fake git-daemon' '
+	mkdir remote &&
+	git init --bare remote/one.git &&
+	mkdir remote/host &&
+	git init --bare remote/host/two.git &&
+	write_script fake-daemon <<-\EOF &&
+	git daemon --inetd \
+		--informative-errors \
+		--export-all \
+		--base-path="$TRASH_DIRECTORY/remote" \
+		--interpolated-path="$TRASH_DIRECTORY/remote/%H%D" \
+		"$TRASH_DIRECTORY/remote"
+	EOF
+	export TRASH_DIRECTORY &&
+	PATH=$TRASH_DIRECTORY:$PATH
+'
+
+test_expect_success 'ext command can connect to git daemon (no vhost)' '
+	rm -rf dst &&
+	git clone "ext::fake-daemon %G/one.git" dst
+'
+
+test_expect_success 'ext command can connect to git daemon (vhost)' '
+	rm -rf dst &&
+	git clone "ext::fake-daemon %G/two.git %Vhost" dst
+'
+
 test_done