Normalize paths in nvm_tree_contains_path before comparing

[gridbugs]

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.

[gridbugs]

This will not work on macos because macos doesn't come with realpath (it's part of coreutils which can be installed with brew). I'm opening this PR to get people's thoughts.

[ljharb]

The lack of realpath in POSIX (and thus on Macs) is why this hasn't been added before, which is why #617 remains a problem. I'd like a solution, but there has to be a fallback for when realpath is not available, or else it won't be viable.

[ljharb]

@stevebob turns out cd -P is in POSIX, so that might be a solution here? (#2143)

[GolanTrevize10]

What about a function like this

realpath() { OURPWD=$PWD cd "$(dirname "$1")" LINK=$(readlink "$(basename "$1")") while [ "$LINK" ]; do cd "$(dirname "$LINK")" LINK=$(readlink "$(basename "$1")") done REALPATH="$PWD/$(basename "$1")" cd "$OURPWD" echo "$REALPATH" }

[ljharb]

Is readlink in posix?

[GolanTrevize10]

the readlink function yes, the readlink command no

[GolanTrevize10]

As @ljharb suggested this would be POSIX
cd -P "$NVM_DIR" && pwd

[mvastola]

What about replacing readlink with node -e "console.log(require('fs').realpathSync(process.env.HOME));"?

At the time nvm_tree_contains_path is called, the PATH should be updated to contain node..

[ljharb]

@mvastola not every node version has fs.realPathSync.

[gridbugs]

The problem this PR solves no longer occurs (just tested with v0.38). Closing.