help: add documentation on include: and subinclude: Adds some help text about the new include: and subinclude: match syntax.

match: enable 'subinclude:' syntax This adds a new rule syntax that allows the user to include a pattern file, but only have those patterns match against files underneath the subdirectory of the pattern file. This is useful when you have nested projects in a repository and the inner projects wants to set up ignore rules that won't affect other projects in the repository. It is also useful in high commit rate repositories for removing the root .hgignore as a point of contention.

copies: switch to using pathutil.dirname copies had it's own dirname implementation. Now that pathutils has a common one, let's use that instead.