Add a smart tooltip to show descriptions in the diffviewer

Review Request #7256 — Created April 27, 2015 and submitted — Latest diff uploaded

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Review Board
dvcs
fe4fb6d...

Reviewers

Hovering over a summary in the commit description table now shows a
tooltip with copyable text if the commit's description is longer than
he summary. Moving the mouse off of the tooltip causes it to disappear.

Verified that the description was correctly shown.

The tooltip's text align's pixel-perfectly with the table's text in
Chrome, Firefox, and Safari on OS X.