sequencer: introduce functions to handle autostashes via refs

sequencer: introduce functions to handle autostashes via refs

We're about to convert the MERGE_AUTOSTASH ref to become non-special,
using the refs API instead of direct filesystem access to both read and
write the ref. The current interfaces to write autostashes is entirely
path-based though, so we need to extend them to also support writes via
the refs API instead.

Ideally, we would be able to fully replace the old set of path-based
interfaces. But the sequencer will continue to write state into
"rebase-merge/autostash". This path is not considered to be a ref at all
and will thus stay is-is for now, which requires us to keep both path-
and refs-based interfaces to handle autostashes.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>