t7610: clean up foo.XXXXXX tmpdir

t7610: clean up foo.XXXXXX tmpdir

The lazy prereq for MKTEMP uses "mktemp -t" to see if
mergetool's internal mktemp call will be able to run. But
unlike the call inside mergetool, we do not ever bother to
clean up the result, and the /tmp of git developers will
slowly fill up with "foo.XXXXXX" directories as they run the
test suite over and over.  Let's clean up the directory
after we've verified its creation.

Note that we don't use test_when_finished here, and instead
just make rmdir part of the &&-chain. We should only remove
something that we're confident we just created. A failure in
the middle of the chain either means there's nothing to
clean up, or we are very confused and should err on the side
of caution.

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