git-gui: Display error dialog on Mac OS X when no .git found.

git-gui: Display error dialog on Mac OS X when no .git found.

If we can't locate a .git directory for the given directory we need to
show a message to the user to let them know the directory wasn't found.
But since this is before we have shown our main application window we
cannot use that as the parent for the error popup; on Mac OS X this
causes an error and prevents the dialog from showing.

Instead only add -parent . to the popup call if we have mapped (shown)
the main window.

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