t4060: make test work with SHA-256

In this test, there are two main types of object IDs we see in the
diffs: the ones for the submodules, which we care about, and the ones
for the individual files, which are unrelated to what we're testing.
Much of the test already computes the former, so extend the rest of the
test to do so as well.  Add a diff comparison function that normalizes
the differences in the latter, since they're not explicitly what we're
testing.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204