Bug: GUI gets threading wrong if mail happens to have same subject

Two e-mail threads with the same subject: 'mod_lua and subprocess_env' [1]

In the threaded and treeview modes, they are shown as part of the same thread (but they are not).

However if one clicks the individual Permalinks [2,3] the threading works properly.

[1] https://lists.apache.org/list.html?users@httpd.apache.org:2018-1:subprocess_env
[2] https://lists.apache.org/thread.html/31613d3c7a88a41ef9bf18df1a3c0e77d7eeda4b2d79f747ec9210b7@%3Cusers.httpd.apache.org%3E
[3] https://lists.apache.org/thread.html/c2a5654554abb620338190900ebc362b44d1336d9b15cecca73b395b@%3Cusers.httpd.apache.org%3E

Start by reproducing the two linked mailing-list examples in the GUI's threaded and treeview modes, then compare them with the individual permalink views. The issue is done when messages with the same subject remain in their distinct threads in both views.

Written by the indexing model from the issue text.