hgweb: display blamed revision once per block in annotate view I.e. when a revision blames a block of source lines, only display the revision link on the first line of the block (this is identified by the "blockhead" key in annotate context). This addresses item "Visual grouping of changesets" of the blame improvements plan (https://www.mercurial-scm.org/wiki/BlamePlan) which states: "Typically there are block of lines all attributed to the same revision. Instead of rendering the revision/changeset for every line, we could only render it once per block."