Don't clobber path attribute if it exists already.

This fixes a bug introduced in #d280d417ea where top-of-file comments on non-changeset views were vanishing after page reload.
CSS cleanup and add row hover

Adds hover background-color to match the single-file view, moves the TH height adjustment into the CSS file.
Hover over row, not TH cell; fix dialog box title on changeset line comment

* Display the comment bubble when hovering anywhere on the row.
* Fix an issue where add comment box was not displaying that the comment would be added to a changeset, not a file.