t7610: fix flaky timeout issue, don't clone from example.com

t7610: fix flaky timeout issue, don't clone from example.com

When t7610-mergetool.sh runs without failures the git://example.com
submodule URLs will never be used. That's because we "git submodule
add" it, but then manually populate them so that subsequent "git
submodule update -N" won't attempt to clone it, only update it without
fetching.

But if we fail in an earlier test it'll have the knock-on effect of
having later tests hang on that "git submodule update -N" as we
attempt to clone this repository from example.com.

This can be reproduced on "master" by running the test with
SANITIZE=leak without "--immediate". With
"GIT_TEST_PASSING_SANITIZE_LEAK=true" (which the linux-leaks job uses)
we'll skip the test entirely. So we'll only run into this when running
it manually, or with the "GIT_TEST_PASSING_SANITIZE_LEAK=check" mode.

That's not because the failure has anything to do with leak detection
per-se. It just so happens that we have a leak that'll fail before
we've managed to fully set these up, and therefore "git submodule
update -N" ends up spawning "git clone".

Let's instead continue lying about the origin of this submodule by
providing a URL for it that doesn't work, but now one that *really*
doesn't work: /dev/null. If the test is passing we won't ever use
this, and if we have knock-on failures we'll fail early, instead of
waiting for a timeout.

The behavior of "-N" here might be surprising to some, since it's
explained as "[if you use -N we] don’t fetch new objects from the
remote site". But (perhaps counter-intuitively) it's only talking
about if it needs to do so via "git fetch". In this case we'll end up
spawning a "git clone", as we have no submodule set up.

See ff7f089ed10 (mergetool: Teach about submodules, 2011-04-13) for
the commit that implemented these "example.com" tests.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>