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.