git-gui: Handle Ctrl + BS/Del in the commit msg

git-gui: Handle Ctrl + BS/Del in the commit msg

- Control+BackSpace: Delete word to the left of the cursor.
- Control+Delete   : Delete word to the right of the cursor.

Originally introduced by BRIEF and Turbo Vision between 1985 and 1992,
they were adopted by most CUA-Compliant UIs, including those of: OS/2,
Windows, Mac OS, Qt, GTK, Open/Libre Office, Gecko, and GNU Emacs.

In both cases Tk already implements the functionality bound to other key
combination, so we use that.

Graphical examples:

Deleting to the left:
        v------ pointer
X_WORD____X
  ^-----^------ selection

Deleting to the right:
  v--------- pointer
X_WORD_X
  ^--^------ selection

Signed-off-by: Ismael Luceno <ismael.luceno@tttech-auto.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>