Fix copying diffs as rich text

[avit]

Possibly related to #111 and #123, in the older (brotherbard) fork I was able to copy diffs and paste them as nicely formatted tables into Mail.app. This doesn't seem to work anymore: it crashes Mail.app.

However, I'm not sure if this was broken in the laullon fork, or in my update to Lion with the new Mail.app... (But other rich text pasting still works in Mail.app.)