releasenotes: command to manage release notes files Per discussion on the mailing list, we want better release notes for Mercurial. This patch introduces an extension that provides a command for producing release notes files. Functionality is implemented as an extension because it could be useful outside of the Mercurial project and because there is some code (like rst parsing) that already exists in Mercurial and it doesn't make sense to reinvent the wheel. The general idea with the extension is that changeset authors declare release notes in commit messages using rst directives. Periodically (such as at publishing or release time), a project maintainer runs `hg releasenotes` to extract release notes fragments from commit messages and format them to an auto-generated release notes file. More details are explained inline in docstrings. There are several things that need addressed before this is ready for prime time: * Moar tests * Interactive merge mode * Implement similarity detection for individual notes items * Support customizing section names/titles * Parsing improvements for bullet lists and paragraphs * Document which rst primitives can be parsed * Retain arbitrary content (e.g. header section/paragraphs) from existing release notes file * Better error messages (line numbers, hints, etc)