i18n: merge-recursive: mark strings for translation

Mark strings in merge-recursive for translation.

Some tests would start to fail with GETTEXT_POISON turned on after
this update.  Use test_i18ncmp and test_i18ngrep where appropriate
to mark strings that should only be checked in the C locale output
to avoid such issues.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Reviewed-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>