gitk: Make text selectable on Mac

gitk: Make text selectable on Mac

Stolen from git-gui, 23effa79f7 (original log message by
Shawn O. Pearce <spearce@spearce.org> follows):

    git-gui: Force focus to the diff viewer on mouse click.

    Apparently a "feature" of Tcl/Tk on Mac OS X is that a disabled text
    widget cannot receive focus or receive a selection within it.  This
    makes the diff viewer almost useless on that platform as you cannot
    select individual parts of the buffer.

    Now we force focus into the diff viewer when its clicked on with
    button 1.  This works around the feature and allows selection to
    work within the viewer just like it does on other less sane systems,
    like Microsoft Windows.

Signed-off-by: Stefan Haller <stefan@haller-berlin.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>