If we are opening an error dialog we want it to appear above all of
the other windows, even those that we may have opened with a grab
to make the window modal. Failure to do so may allow an error
dialog to open up (and grab focus!) under an existing toplevel,
making the user think git-gui has frozen up and is unresponsive,
as they cannot get to the dialog.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>