This fixes the bug reported in
id:6F2EF901-8B4B-44FF-83C5-22F732BA95A6@gmail.com
Unfortunately it turns out our test data has several tabs in the
subject lines. The expected output was updated to reflect their
removal and the ripple effect of several more subjects matching the
previous ones.
With this mode, one can fold trees in the notmuch-tree buffer as if
they were outlines, using all the commands provided by
outline-minor-mode. We also define a couple of movement commands
that, optional, will ensure that only the thread around point is
unfolded.
The implementation is based on registering a :level property in the
messages p-list, that is then used by outline-minor-mode to to
recognise headers.
Amended by db: Copy docstring to manual and edit for presentation. Add
two tests. Fix typo "wether".
