t5410: use longer path for sample script

t5410 creates a sample script "alternate-refs", and sets
core.alternateRefsCommand to just "alternate-refs". That
shouldn't work, as "." is not in our $PATH, and so we should
not find it.

However, due to a bug in run-command.c, we sometimes find it
anyway! Even more confusing, this bug is only in the
fork-based version of run-command. So the test passes on
Linux (etc), but fails on Windows.

In preparation for fixing the run-command bug, let's use a
more complete path here.
Reported-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5410-receive-pack-alternates.sh

@@ -23,7 +23,7 @@ test_expect_success 'with core.alternateRefsCommand' '
 			--format="%(objectname)" \
 			refs/heads/public/
 	EOF
-	test_config -C fork core.alternateRefsCommand alternate-refs &&
+	test_config -C fork core.alternateRefsCommand ./alternate-refs &&
 	git rev-parse public/branch >expect &&
 	printf "0000" | git receive-pack fork >actual &&
 	extract_haves <actual >actual.haves &&