hgweb: add a floating tooltip to invite on followlines action
In followlines.js, we create a <div id="followlines-tooltip"> element to draw
attention of users on "followlines" feature. The element shows up on hover of
source lines after one second and follows the cursor. After first click (start
line selection), the text changes and indicates that next click will terminate
selection.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.