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>

t/t8006-blame-textconv.sh

@@ -73,6 +73,28 @@ test_expect_success 'blame --textconv going through revisions' '
 	test_cmp expected result
 '
 
+test_expect_success 'setup +cachetextconv' '
+	git config diff.test.cachetextconv true
+'
+
+cat >expected_one <<EOF
+(Number2 2010-01-01 20:00:00 +0000 1) converted: test 1 version 2
+EOF
+
+# one.bin is blamed as 'Not Committed yet'
+test_expect_failure 'blame --textconv works with textconvcache' '
+	git blame --textconv two.bin >blame &&
+	find_blame <blame >result &&
+	test_cmp expected result &&
+	git blame --textconv one.bin >blame &&
+	find_blame  <blame >result &&
+	test_cmp expected_one result
+'
+
+test_expect_success 'setup -cachetextconv' '
+	git config diff.test.cachetextconv false
+'
+
 test_expect_success 'make a new commit' '
 	echo "bin: test number 2 version 3" >>two.bin &&
 	GIT_AUTHOR_NAME=Number3 git commit -a -m Third --date="2010-01-01 22:00:00"