We don't want to do this in the commit view, as that way you can't
commit whitespace differences. You'll never be able to have a clean
working tree, and you can't see why the files remain 'unstaged'. So, we do this
only for the history view :)
Since Git recommends to use 50 characters in the first line of a commit
message, this adds a vertical line to the commit message view to show where
the 50 char limit is.
[PB: This preference can be changed using a user default -- for now we don't
have a preference pane yet, so this is hidden.]
Stonewall Ballard