Implement evn.path.extract_component_name().
Improve CopyOrRename.* logic for component repos.
Add TestMultiComponentRepo2; tests copy/rename root path protection for component repos.
Tweak error message re-writing for component-specific errors.
Make sure single->multi conversion is handled gracefully.
Initial implementation of core branching/tagging tests.
Merge branch 'issue24-known-root-to-known-root-subpath-broken'.

First-pass fixes for #24, #25.