fixing Metadata.relative_path for /foo/bar , /foo/bar_baz becoming ._baz

The behaviour of relative path in Metadata was causing me problems when invoking rspec in a directory named
/foo/bar, while the tests were in /foo/bar_baz.

Metadata.relative_path solved them to ._baz, instead of seeing that the tests are not in the directory of invocation, and using the absolute path instead.

Not sure why this is done with regexes though. The pathname object supports finding the relative path from one to the other.
Can you make this a regex literal rather than `Regexp.new`?  That fits with the style we use everywhere else.

Also, I'm not sure I totally understand this regexp yet.  For non-trivial regexes I usually make an example on http://rubular.com/, make a permalink for it, and add a comment in the source with that permalink so we can revisit the rubular example later.  Mind doing that?