Completion for ":cd" from 'cdpath'

[GoogleCodeExporter]

The `cd` command should consider the `cdpath` setting for completion of 
relative paths.

Currently the completion appears to use only the current working directory when 
completing relative paths.

This means that `:cd foo<Tab>` should complete `foobar` if there exists 
`/path/in/cdpath/foobar` (and `/path/in/cdpath` is in `cdpath`).

Original issue reported on code.google.com by dhahler@gmail.com on 15 Jun 2015 at 6:55