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.