msglist: fix inconsistent thread subject deduplication

When threading is enabled, some messages which have different base
subjects are hidden whereas some others which have similar subjects are
displayed:

 [PATCH aerc v3 1/3] mk: remove smart rebuild when GOFLAGS have changed
 ├─>[PATCH aerc v3 2/3] mk: only install changed/missing files
 │  └─>Re: [PATCH aerc v3 2/3] mk: only install changed/missing files
 │     └─>Re: [PATCH aerc v3 2/3] mk: only install changed/missing files
 ├─>
 └─>Re: [PATCH aerc v3 1/3] mk: remove smart rebuild when GOFLAGS have changed

This happens because we are using the "previous" message to get the
subject base instead of the current one. Compute the base subject from
the current message. Here is the result:

 [PATCH aerc v3 1/3] mk: remove smart rebuild when GOFLAGS have changed
 ├─>[PATCH aerc v3 2/3] mk: only install changed/missing files
 │  └─>
 │     └─>
 ├─>[PATCH aerc v3 3/3] mk: speed up notmuch detection
 └─>Re: [PATCH aerc v3 1/3] mk: remove smart rebuild when GOFLAGS have changed

Fixes: 535300cfdbfc ("config: add columns based index format")
Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

0 files changed, 0 insertions, 0 deletions