hgweb: parameterize the tag name of elements holding followlines selection While plugging followlines.js into "annotate" view, we'll need to walk a different DOM structure from that of "filerevision" view. In particular, the selectable source line element is a <tr> in annotate view (in contrast with a <span> in filerevision view). So make this tag name a parameter of followlines.js script by passing its value as a "selectabletag" data attribute of <pre class="sourcelines"> element. As <pre class="sourcelines"> tags are getting quite long in templates, rewrite them on several lines.