hgweb: expose a followlines UI in filerevision view In filerevision view (/file/<rev>/<fname>) we add some event listeners on mouse clicks of <span> elements in the <pre class="sourcelines"> block. Those listeners will capture a range of lines selected between two mouse clicks and a box inviting to follow the history of selected lines will then show up. Selected lines (i.e. the block of lines) get a CSS class which make them highlighted. Selection can be cancelled (and restarted) by either clicking on the cancel ("x") button in the invite box or clicking on any other source line. Also clicking twice on the same line will abort the selection and reset event listeners to restart the process. As a first step, this action is only advertised by the "cursor: cell" CSS rule on source lines elements as any other mechanisms would make the code significantly more complicated. This might be improved later. All JavaScript code lives in a new "linerangelog.js" file, sourced in filerevision template (only in "paper" style for now).