git-gui: Fix 'Select All' action on Windows.

git-gui: Fix 'Select All' action on Windows.

Sometimes the Select All action from our context menus doesn't work
unless the text field its supposed to act on has focus.  I'm not
really sure why adding the sel tag requires having focus.  It
technically should not be required to update the sel tag membership,
but perhaps there is a bug in Tcl/Tk 8.4.1 on Windows which is
causing this odd behavior.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>