More robust implementation of: Do not spell-check svn status (added, modified, ...) lines.

I had submitted parts of that in #2 a year ago, but you chose to implement this instead with a dedicated `svnText` group. Now, I've found problems with your (rather unusual) implementation: When using _compound filetypes_ (e.g. `:setf svn.txt`, with a `txt` syntax providing additional highlighting for bold and italic), it won't affect the text due to the `svnText` containment, and the `containedin=ALL` wreaks havoc with the other syntax.

Therefore, get rid of `svnText` and use `@NoSpell` instead.

As a bonus, I've added a separate highlighting for the unimportant separator boilerplate line.