add userdiff textconv tests

These tests provide a basic sanity check that textconv'd
files work. The tests try to describe how this configuration
_should_ work; thus some of the tests are marked to expect
failure.

In particular, we fail to actually textconv anything because
the 'diff.foo.binary' config option is not set, which will
be fixed in the next patch.

This also means that some "expect_failure" tests actually
seem to be fixed; in reality, this is just because textconv
is broken and its failure mode happens to make these tests
work.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>