t4013: make test hash independent

t4013: make test hash independent

This test produces a large number of diff formats and compares the
output with test files that have content specific to SHA-1. Since we are
more interested in the format of the diffs, and not their specific
values, which are tested elsewhere, add a function which uses sed to
transform these specific object IDs into generic ones of the right size,
which we can then compare.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>