thread: use hash + array instead of hand-rolled linked list

This starts to show noticeable performance improvements when
attempting to thread over 400 messages; but the improvement
may not be measurable with less.

However, the resulting code is much shorter and (IMHO)
much easier to understand.

1 files changed, 1 insertions, 0 deletions

diff --git a/MANIFEST b/MANIFEST
index bcc41216..3a4d7c4d 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -155,6 +155,7 @@ t/qspawn.t
 t/search.t
 t/spamcheck_spamc.t
 t/spawn.t
+t/thread-cycle.t
 t/utf8.mbox
 t/view.t
 t/watch_maildir.t