Currently the thread is named based on either the oldest or newest
matching message (depending on the search order). If this message has
an empty subject, though, the thread will show up with an empty
subject in the search results. (See the thread starting with
`id:1412371140-21051-1-git-send-email-david@tethera.net` for an
example.)

This patch changes the behavior to name based on the oldest/newest
matching non-empty subject. This is particularly helpful for patchsets.
If the only subjects are empty, the thread subject will still be empty.

Signed-off-by: Jesse Rosenthal <jrosenthal@jhu.edu>
---
 lib/thread.cc | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/thread.cc b/lib/thread.cc
index 8922403..ea10295 100644
--- a/lib/thread.cc
+++ b/lib/thread.cc
@@ -348,18 +348,20 @@ _thread_add_matched_message (notmuch_thread_t *thread,
 {
     time_t date;
     notmuch_message_t *hashed_message;
+    const char *cur_subject;
 
     date = notmuch_message_get_date (message);
+    cur_subject = notmuch_thread_get_subject (thread);
 
     if (date < thread->oldest || ! thread->matched_messages) {
 	thread->oldest = date;
-	if (sort == NOTMUCH_SORT_OLDEST_FIRST)
+	if (sort == NOTMUCH_SORT_OLDEST_FIRST || strlen(cur_subject) == 0)
 	    _thread_set_subject_from_message (thread, message);
     }
 
     if (date > thread->newest || ! thread->matched_messages) {
 	thread->newest = date;
-	if (sort != NOTMUCH_SORT_OLDEST_FIRST)
+	if (sort != NOTMUCH_SORT_OLDEST_FIRST || strlen(cur_subject) == 0)
 	    _thread_set_subject_from_message (thread, message);
     }
 
-- 
2.1.2