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>