highlight: exit early on textual and unknown files (issue3005) When highlight extension encountered files that pygments didn't recognize, it used to fall back to text lexer. Also, pygments uses TextLexer for .txt files. This lexer is noop by design. On bigger files, however, doing the noop highlighting resulted in noticeable extra CPU work and memory usage: to show a 1 MB text file, hgweb required about 0.7s more (on top of ~3.8s, Q8400) and consumed about 100 MB of RAM more (on top of ~150 MB). Let's just exit the function when it's clear that nothing will be highlighted. Due to how this pygmentize function works (it modifies the template in-place), we can just return from it and everything else will work as if highlight extension wasn't enabled.