It's possible that `tree` and `node_path` both refer to the same
directory, but are lexically different. This converts both paths
into a canonical form so that any lexical-only differences are
removed.
This solves a problem where setting `NVM_DIR` to /home/user//.nvm
instead of /home/user/.nvm results in the infamous:
nvm is not compatible with the npm config "prefix" option
This adds a test that the above case no longer results in an error. An
existing test stopped failing as expected as a result, as the test set
the npm prefix to a local-directory-relative path, which resolved to
a directory inside the `NVM_DIR` when running the tests from within the
`NVM_DIR`. This test is updated to use an absolute path.
