Newly added tests to t3420 in this series prepare expected
human-readable output from "git rebase -i" and then compare the
actual output with it.  As the output from the command is designed
to go through i18n/l10n, we need to use test_i18ncmp to tell
GETTEXT_POISON build that it is OK the output does not match.

Signed-off-by: Junio C Hamano <gitster@pobox.com>