git-gui: Don't crash in ask_popup if we haven't mapped main window yet

git-gui: Don't crash in ask_popup if we haven't mapped main window yet

If we have more than our desired number of objects and we try to
open the "Do you want to repack now?" dialog we cannot include a
-parent . argument if the main window has not been mapped yet.
On Mac OS X it appears this window isn't mapped right away, so we
had better hang avoid including it.

Signed-off-by: Shawn O. Pearce <redacted>