Add subtree capability using :repo_tree variable

The variable `:repo_tree` now allows the specification of a sub-tree
that will be extracted from the repository when deploying. This is
useful when deploying a project that lives in a subdirectory of a larger
repository.  Implemented only for git and hg.  If not defined then the
behaviour is as previously and the whole repository is extracted.
All extraction is as before still subject to git-archive `.gitattributes`
Thanks to you both, I <3 to see people playing well together on FOSS. Good luck with your deployments @scottsuch, and @townsen as ever, thank you sincerely for your continuing valuable contributions to the project!