* (bug 1438) Fix for diff table layout on very wide lines.

* (bug 1438) Fix for diff table layout on very wide lines.
  Diff style rules have been broken out to common/diff.css,
  and the dupes removed from the default skin files.
  Skins can still override the default rules.

Improvements over r22192, now known to work in:
* Firefox 2.0.0.3
* Opera 9.10 and 9.20
* Safari 2.0.4
* Konqueror 3.5.6
* MSIE/Win 7.0 (wide cells may produce vertical scrollbars as well)
* MSIE/Win 6.0 (wide cells are cropped instead of scrolling)
* MSIE/Mac 5.2.3 (wide words break instead of scrolling)
* iCab 3.0.3 (some cells provoke unnecessary horizontal scrollbar)

I've cleaned up the diff table formatting a bit, moving some attributes
from the HTML to the style sheet and consolidating the duplicated styles
into a common/diff.css file which is conditionally loaded for diff views.

Individual skins or site/user CSS can still override that style if they wish.