minor enhancement to opacity cssHooks

[lrbabe]

Why are we doing

var filter = style.filter || "";
style.filter = style.filter + ' ' + opacity;

When we could do

var filter = style.filter || "";
// reuse the 'filter' var
style.filter = filter + ' ' + opacity;

Improve jQuery, one DOM access at a time.

The actual code is slightly longer, but the modification is equally tiny, see following push request.

[lrbabe]

Here is the pull request:  https://github.com/jquery/jquery/pull/254/files

[john]

Landed.