highlight: add highlightfiles config option which takes a fileset (issue3005) Highlight extension lacked a way to limit files by size, by extension, and/or by any other part of file path. A good solution would be to use a fileset, since it can check file path, extension and size (and more) in one expression. So this change introduces such an option, highlighfiles, which takes a fileset and on each request decides if the requested file should be highlighted. The default "size('<5M')" is, in a way, suggested in issue3005. checkfctx() limits the amount of work to just one file (subset kwarg in fileset.matchctx()). Monkey-patching works around issue4568, otherwise using filesets here while running hgweb in directory mode would say, for example, "Abort: **.py not under root", but this fix is very local and probably far from ideal. I suspect there to be a way to fix this for the whole hgweb and resolve the issue, but I don't know how to do it.