hgweb: side-by-side comparison functionality Adds new web command to the core, ``comparison``, which enables colorful side-by-side change display, which for some might be much easier to work with than the standard line diff output. The idea how to implement comes from the SonicHq extension. The web interface gets a new link to call the comparison functionality. It lets users configure the amount of context lines around change blocks, or to show full files - check help (also in this changeset) for details and defaults. The setting in hgrc can be overridden by adding ``context=<value>`` to the request query string. The comparison creates addressable lines, so as to enable sharing links to specific lines, just as standard diff does. Incorporates updates to all web related styles. Known limitations: * the column diff is done against the first parent, just as the standard diff * this change allows examining diffs for single files only (as I am not sure if examining the whole changeset in this way would be helpful) * syntax highlighting of the output changes is not performed (enabling the highlight extension has no influence on it)