wt-status: replace sha1 mentions with oid

wt-status: replace sha1 mentions with oid

`abbrev_sha1_in_line()` uses a `struct object_id oid` and should be
fully prepared to handle non-SHA1 object ids. Rename it to
`abbrev_oid_in_line()`.

A few comments in `wt_status_get_detached_from()` mention "sha1". The
variable they refer to was renamed in e86ab2c1cd ("wt-status: convert to
struct object_id", 2017-02-21). Update the comments to reference "oid"
instead.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>