hgweb: render next pages on /graph incrementally Previously, when user scrolled down to see the next page on /graph, all hgweb did was re-render everything that would be visible (by simply incrementing revcount). It was not efficient at all, and this patch makes /graph page behave similarly to the regular /log: every new page only consists of new changesets, no duplication, and only jsdata is based on the full set of changesets required to build accurate graph. This is achieved by adding "?graphtop=<node>" to the next page URL template, effectively remembering where the graph started, and using that value to create the new `tree` that covers the whole visible graph. That variable is then used to produce jsdata for redrawing graph client-side. nextentry is used for the same purpose as on /log page (to format the next page URL), but it's not a part of the graph.

hgweb: split graphdata() into jsdata() and nodes() nodes keyword passed to the template can be any iterator, but jsdata needs to be a list because it gets JSONified.

hgweb: calculate <canvas> width and height client-side hgweb determines and passes to templates some variables related to graph appearance, like bg_height, canvaswidth and canvasheight. bg_height was and still is used for graph.scale() call in graph.tmpl, and the two latter variables were used in <canvas> element as width and height properties, and they were set before JS code got to run. Setting these properties server-side doesn't make a lot of sense, because a graph that has been scaled should calculate things like width and height on its own when being rendered. Let's move (re)sizing <canvas> to JavaScript (to Graph.render function) and stop parsing HTML with regular expressions just to know new width and height. That extra loop that only counts cols is required because <canvas> can't be resized after or in the process of rendering (or it gets cleared). Incidentally, SVG doesn't have this problem and I'm hoping to switch graph to using it in future. There also was truecanvasheight, but according to hg grep --all it was never used, see d490edc71146.

hgweb: filter graphmod.colored() output before iterating over it Consumers in this function use output of graphmod.colored(), but only want items with type == CHANGESET, so let's filter it early. This is primarily just a refactoring, but it also fixes a potential small bug with `rows = len(tree)` (this variable is used for "Rows shown" line in raw-graph) if there are items of other types.