t5570: Update for clone-progress-to-stderr branch

git clone now reports its progress to standard error, which throws off
t5570.  Using test_i18ngrep instead of test_cmp allows the test to be
more flexible by only looking for the expected error and ignoring any
other output from the program.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>