t1300-repo-config: make it resilient to being run via 'sh -x'

t1300-repo-config: make it resilient to being run via 'sh -x'

One way to diagnose broken regression tests is to run the test
script using 'sh -x t... -i -v' to find out which call actually
demonstrates the symptom.

Hence it is pretty counterproductive if the test script behaves
differently when being run via 'sh -x', in particular when using
test_cmp or test_i18ncmp on redirected stderr.  A more recent way
"sh tXXXX -i -v -x" has the same issue.

So let's use test_i18ngrep (as suggested by Jonathan Nieder) instead of
test_cmp/test_i18ncmp to verify that stderr looks as expected.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>