t7615: be more explicit about diff algorithm used

t7615: be more explicit about diff algorithm used

t7615 is entirely about testing the differences about different
diff algorithms, but it doesn't specify any diff algorithm when it
is testing myers.  Given that we have discussed potentially switching
defaults (https://lore.kernel.org/git/xmqqed873vgn.fsf@gitster.g/), it
makes sense in tests that are about different diff algorithms to be
explicitly about which one is intended to be used in each test.  Add
that specificity.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>