highlight: add option to prevent content-only based fallback When Mozilla enabled Pygments on hg.mozilla.org, we got a lot of weirdly colorized files. Upon further investigation, the hightlight extension is first attempting a filename+content based match then falling back to a purely content-driven detection mode in Pygments. Sounds good in theory. Unfortunately, Pygments' content-driven detection establishes no minimum threshold for returning a lexer. Furthermore, the detection code for a number of languages is very liberal. For example, ActionScript 3 will return a confidence of 0.3 (out of 1.0) if the first 1k of the file we pass in matches the regex "\w+\s*:\s*\w"! Python matches on "import ". It's no coincidence that a number of our extension-less files were getting highlighted improperly. This patch adds an option to have the highlighter not fall back to purely content-based detection when filename+content detection failed. This can be enabled to render unlighted text instead of taking the risk that unknown file types are highlighted incorrectly. The old behavior is still the default.

highlight: inline checkfctx() It is only used once. pygmentize() is pretty small. Let's just inline it.