sequencer: print autostash messages to stderr

sequencer: print autostash messages to stderr

The rebase messages are printed to stderr traditionally. However due
to a bug introduced in 587947750bd (rebase: implement --[no-]autostash
and rebase.autostash, 2013-05-12) which was faithfully copied when
reimplementing parts of the interactive rebase in the sequencer the
autostash messages are printed to stdout instead.

It is time to fix that: let's print the autostash messages to stderr
instead of stdout.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Phillip Wood <redacted>
Signed-off-by: Junio C Hamano <redacted>