t/lib-rebase: set_fake_editor(): handle FAKE_LINES more consistently

t/lib-rebase: set_fake_editor(): handle FAKE_LINES more consistently

Default next action after 'fakesha' to preserving the command instead
of forcing 'pick', consistently with other "instant-effect" keywords.
There is no reason why one would want that inconsistency, so this was
clearly just an oversight in commit 5dcdd740 ("t/lib-rebase: prepare
for testing `git rebase --rebase-merges`"). Rectifying it makes the
behavior easier to reason about and document.

This would affect hypothetical "fakesha <n>" sequences where line <n>
already isn't a pick, which currently don't appear.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>