tmux 2.8 has a broken version string (2.8-rc) which we patch in configure.ac
CHANGES FROM 2.7 to 2.8
* Make display-panes block the client until a pane is chosen or it
times out.
* Clear history on RIS like most other terminals do.
* Add an "Any" key to run a command if a key is pressed that is not
bound in the current key table.
* Expand formats in load-buffer and save-buffer.
* Add a rectangle_toggle format.
* Add set-hook -R to run a hook immediately.
* Add README.ja.
* Add pane focus hooks.
* Allow any punctuation as separator for s/x/y not only /.
* Improve resizing with the mouse (fix resizing the wrong pane in some
layouts, and allow resizing multiple panes at the same time).
* Allow , and } to be escaped in formats as #, and #}.
* Add KRB5CCNAME to update-environment.
* Change meaning of -c to display-message so the client is used if it
matches the session given to -t.
* Fixes to : form of SGR.
* Add x and X to choose-tree to kill sessions, windows or panes.
4e75120cd4