i18n: mark checkout plural warning for translation

i18n: mark checkout plural warning for translation

Mark the "Warning: you are leaving %d commit(s) behind" message added
in v1.7.5-rc0~74^2 (commit: give final warning when reattaching HEAD
to leave commits behind) by Junio C Hamano for translation.

This message requires the use of ngettext() features, and is the first
message to use the Q_() wrapper around ngettext().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>