tests: do not use implicit "git diff --no-index"

tests: do not use implicit "git diff --no-index"

As a general principle, we should not use "git diff" to validate the
results of what git command that is being tested has done.  We would not
know if we are testing the command in question, or locating a bug in the
cute hack of "git diff --no-index".

Rather use test_cmp for that purpose.

Signed-off-by: Junio C Hamano <gitster@pobox.com>