I would recommend backing out your commit (hg rollback if you haven't pulled or otherwise changed your checkout since you made your commit), and just do 'hg diff' at the point where you would commit.  In this particular case, if there are no changes other than the added file it doesn't really matter much that it's not in patch form.

It would be nice to have some unit tests.  It may not be possible to test anything but your _str_to_font and _font_to_str functions, though.

I notice that your Fontchooser class doesn't inherit from commondialog.Dialog like colorchooser.Chooser does; is commondialog.Dialog usable for this?  Or are there improvements that can be made to commondialog.Dialog to make it suitable for Fontchooser and also improve colorchooser.Chooser?  Can this reuse anything from Lib/tkinter/font.py or perhaps be merged into that file to keep all the font stuff together?

After we have some of those details ironed out, this is going to make a nice addition to tkinter!