Refactor an initialization of a variable added in 03831ef7b50 (difftool: implement the functionality in the builtin,
2017-01-19). This refactoring makes a subsequent change smaller.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Acked-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>