This new command allows the user to interactively choose a different
duplicate (file) to display for a given message in
notmuch-show-mode. Since both tree and unthreaded view use
notmuch-show-mode, this provides the same facility there.
Based on the commit message in id:20210221151902.2301690-3-dme@dme.org
Add the function notmuch-test-result-flags to test-lib.el to avoid
repeating it in 3 T*.sh files.
This is more efficient than notmuch-show-only-matching-messages, since
we do not parse the potentially large thread structure to find a
single message.
This is only a partial fix for notmuch-tree view, because displaying
the thread structure in the tree-mode window still crashes on long
threads. It is however enough to make unthreaded view handle long
threads.
