blame tests: simplify userdiff driver test

blame tests: simplify userdiff driver test

Simplify the test added in 9466e3809d (blame: enable funcname blaming
with userdiff driver, 2020-11-01) to use the --author support recently
added in 999cfc4f45 (test-lib functions: add --author support to
test_commit, 2021-01-12).

We also did not need the full fortran-external-function content. Let's
cut it down to just the important parts.

I'm modifying it to demonstrate that the fortran-specific userdiff
function is in effect by adding "DO NOT MATCH ..." and "AS THE ..."
lines surrounding the "RIGHT" one.

This is to check that we're using the userdiff "fortran" driver, as
opposed to the default driver which would match on those lines as part
of the general heuristic of matching a line that doesn't begin with
whitespace.

The test had also been leaving behind a .gitattributes file for later
tests to possibly trip over, let's clean it up with
"test_when_finished".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>