There are some cases like remote usage where this might cause
problems, but those users can easily customize the variable. The
inconvenience seems to be outweighed by the security benefit for most
users.
When no decryption or signature examination is
happening (i.e. `notmuch-crypto-process-mime' is `nil') insert buttons
that indicate this, rather than remaining silent.
Previously the tags on each line in tree view were separarted by ", "
not just " ". This is different from show and search views.
This patch removes this comma. This is a large patch as essentially
every line of each of the expected outputs in the tree tests needs
updating.
Apart from aesthetic reasons this simplifies the switch to
notmuch-tag-format-tags in the next patch.
