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>

git-gui.sh

@@ -3928,6 +3928,8 @@ bind $ui_comm <$M1B-Key-KP_Subtract> {show_less_context;break}
 bind $ui_comm <$M1B-Key-equal> {show_more_context;break}
 bind $ui_comm <$M1B-Key-plus> {show_more_context;break}
 bind $ui_comm <$M1B-Key-KP_Add> {show_more_context;break}
+bind $ui_comm <$M1B-Key-BackSpace> {event generate %W <Meta-Delete>;break}
+bind $ui_comm <$M1B-Key-Delete> {event generate %W <Meta-d>;break}
 
 bind $ui_diff <$M1B-Key-x> {tk_textCopy %W;break}
 bind $ui_diff <$M1B-Key-X> {tk_textCopy %W;break}