Show column guide in commit message edit dialog

Git has a commit message convention that comment message lines should be manually wrapped to e.g. 72 characters. The SourceTree comment message box has no information about the line length to assist in doing manual wrapping. E.g. GitX has a vertical guide in the edit dialog to show the preferred wrapping column, with a preference setting to hiding the guide and defining the guide column placement. SourceTree should have a similar guide to help in manual line wrapping.

References to line wrapping convention:

http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
https://github.com/torvalds/subsurface/blob/master/README#L161