This test reproduces a regression introduced by commit 97101f1f82
("mu: Prune empty container when an only child gets promoted to the
root set").
When the results of a mu-find query contain only a one thread:
$ mu find --threads --fields 'd s' ''
Sat 09 Aug 2014 07:00:00 PM CEST [mu4e] Test Message
`-> Sat 09 Aug 2014 08:30:00 PM CEST Re: [mu4e] Test Message
... and we narrow down the query in such a way that the root message
gets excluded, then a crash occurs:
$ mu find --threads --fields 'd s' '' date:2014-08-09/20:00..2014-08-09/21:00
**
ERROR:mu-container.c:117:mu_container_append_siblings: assertion failed: (c)
Aborted (core dumped)
Reported-by: Josiah Schwab <jschwab@gmail.com>
