git-gui: Don't delete console window namespaces too early

git-gui: Don't delete console window namespaces too early

If the console finishes displaying its output and is "done" but
needs to draw a scrollbar to show the final output messages it
is possible for Tk to delete the window namespace before it does
the text widget updates, which means we are unable to add the
horizontal or vertical scrollbar to the window when the text
widget decides it cannot draw all glyphs on screen.

We need to delay deleting the window namespace until we know
the window is not going to ever be used again.  This occurs if
we are done receiving output, the command is successful and the
window is closed, or if the window is open and the user chooses
to close the window after the command has completed.

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