Revision header's height is short if Japanese font is selected

[msakuta]

The revision header in the commit log window has insufficient height for its contents.
It shows a vertical scroll bar, but I cannot see the "Parent(s)" line without scrolling.

This issue won't happen when a non-Japanese font is selected in Settings -> Appearance -> Application font. I've confirmed this issue with MS UI Gothic, MS P Gothic and Meiryo. Other multibyte fonts may suffer from the same problem, but I didn't check further.

I've looked into the code and figured out that RichTextBox.GetPreferredSize() has a bug it could return a wrong size. The safe way to fit a RichTextBox's height to its contents is, as described in the page on the following URL, to respond to ContentsResized event.

http://csharphelper.com/blog/2013/07/resize-a-richtextbox-to-fit-its-contents-in-c/

I'll post a pull request for this fix.