Add $sort_thread_groups for top-level thread sorting.

closes #368.

Rename THREAD->sort_key to sort_aux_key, to make the fields more
distinct.  Add THREAD->sort_group_key for caching the top-level sort
HEADER.  An additional value is needed to allow propagating up values,
for place-holder threads, and for "last" sort handling.

Add recalc_aux_key and recalc_group_key bits to THREAD, in addition to
sort_children.  These are used to allow separate flagging of either
the group key or aux key needing to be recalculated.  Adding these two
fields helped me keep better track of how the subthread sorting was
working, because I'm just not as smart the people who originally wrote
the threading code.  :-)

Fix up some of the logic with rescoring, and "first_message" to take
the new config vars into account.

Add %T expando to $status_format, and change the default to
conditionally show the value based on whether $sort is "threads" and
$sort_thread_groups is not "aux".

Lastly, a thank you to Eric Blake.  He implemented separate
thread/subthread sorting in the NeoMutt code, after suggesting it on
the #mutt IRC channel a couple months ago.  When ticket 368 was opened
he pointed me at his work and offered to port.  However, I couldn't
pass up the opportunity to learn more about the thread code myself, so
I declined his offer.

Although no copy/pasting was done, the basic of structure of the
mutt_sort_subthreads() changes is the same, and I did look at his
modifications to the function for guidance.  Thank you Eric!

1 files changed, 4 insertions, 1 deletions

diff --git a/mutt.h b/mutt.h
index 88b71da5..1e7016cc 100644
--- a/mutt.h
+++ b/mutt.h
@@ -952,6 +952,8 @@ struct mutt_thread
   unsigned int fake_thread : 1;
   unsigned int duplicate_thread : 1;
   unsigned int sort_children : 1;
+  unsigned int recalc_aux_key : 1;
+  unsigned int recalc_group_key : 1;
   unsigned int check_subject : 1;
   unsigned int visible : 1;
   unsigned int deep : 1;
@@ -962,7 +964,8 @@ struct mutt_thread
   THREAD *next;
   THREAD *prev;
   HEADER *message;
-  HEADER *sort_key;
+  HEADER *sort_group_key;  /* $sort_thread_groups - for thread roots */
+  HEADER *sort_aux_key;    /* $sort_aux - for messages below the root */
 };