(bug 35923) tweaks to mediawiki.action.history.diff.css

(bug 35923) tweaks to mediawiki.action.history.diff.css

This is a followup from bug 11374. This patch has some fixes and final tweaks
for the code currently in trunk.

* Reorganized the CSS to specificallity.
* Changed table width to 100%, and added zero margin for table, so no
horizontal scrollbars.
* Changed horizontal padding in table.diff td from 0.66em to 0.5em, making max
use of expensive horizontal real estate.
* Override default vertical 0.33em padding in td.diff-marker with 0.25em to
compensate for increased fontsize of the marker (+ and -), which caused
unwanted padding at the bottom in one-line diff rules.
* Changed the background for td.diff-context to #f9f9f9, which is the same as
the background for wikitables. (BTW, the default background for Vector is
#f6f6f6, not the current #f3f3f3 as stated on wikitech-l).
* Removed 'overflow: auto' fallback for table.diff td div, as this creates
unwanted scrollbars in IE 8 and 9. The fallback targeted a very limited
set of older browsers anyway.

The NewDiff gadget on enwiki has all these fixes implemented, so you can
evaluate this code.

Change-Id: Ic9da00bc13ad8c322c19648c7d3ac8abec2eaa07