t/t8006: Demonstrate blame is broken when cachetextconv is on

t/t8006: Demonstrate blame is broken when cachetextconv is on

I have a git repository with lots of .doc and .pdf files. There diff
works ok, but blaming is painfully slow without textconv cache, and with
textconv cache, blame says lots of lines are 'Not Yet Committed' which
is wrong.

Here is a test that demonstrates the problem.

Cc: Axel Bonnet <axel.bonnet@ensimag.imag.fr>
Cc: Clément Poulain <clement.poulain@ensimag.imag.fr>
Cc: Diane Gasselin <diane.gasselin@ensimag.imag.fr>
Cc: Jeff King <peff@peff.net>
Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>