sequencer: mark action_name() for translation

sequencer: mark action_name() for translation

The definition of this function goes back all the way to 043a449
(sequencer: factor code out of revert builtin, 2012-01-11), long before a
serious effort was made to translate all the error messages.

It is slightly out of the context of the current patch series (whose
purpose it is to re-implement the performance critical parts of the
interactive rebase in C) to make the error messages in the sequencer
translatable, but what the heck. We'll just do it while we're looking at
this part of the code.

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