hgweb: add line wrapping switch to file source view This uses classList property, which is well-supported now: both Chromium 8.0+, Firefox 3.6+ and Opera 11.5+ support it, as well as relatively modern versions of other browsers.

hgweb: add changelog and search templates to raw style This will make testing hgweb search and other things simpler and more concise, as there is no extra HTML code in the raw style.

hgweb: toggleDiffstat function instead of showDiffstat and hideDiffstat This eliminates the need of two almost equal functions, makes the code cleaner.