t4001: add a test comparing basename similarity and content similarity

t4001: add a test comparing basename similarity and content similarity

Add a simple test where a removed file is similar to two different added
files; one of them has the same basename, and the other has a slightly
higher content similarity.  In the current test, content similarity is
weighted higher than filename similarity.

Subsequent commits will add a new rule that weighs a mixture of filename
similarity and content similarity in a manner that will change the
outcome of this testcase.

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