nvm is confused by symlinks

[ebiggers]

I cloned the nvm repository into a directory, then sourced nvm.sh via a symbolic link to that directory.

This causes nvm to error out due to incorrect "contains path" logic:

$ nvm use v5.1.0
nvm is not compatible with the npm config "prefix" option: currently set to "/opt/src/nvm/versions/node/v5.1.0"
Run `npm config delete prefix` or `nvm use --delete-prefix v5.1.0` to unset it.

Here's one possible fix:

diff --git a/nvm.sh b/nvm.sh
index b4e871f..7cd1e1d 100755
--- a/nvm.sh
+++ b/nvm.sh
@@ -98,9 +98,9 @@ fi

 nvm_tree_contains_path() {
   local tree
-  tree="$1"
+  tree="$(readlink -f "$1")"
   local node_path
-  node_path="$2"
+  node_path="$(readlink -f "$2")"

   if [ "@$tree@" = "@@" ] || [ "@$node_path@" = "@@" ]; then
     >&2 echo "both the tree and the node path are required"

[ljharb]

This is a duplicate of #617 - for now, $NVM_DIR can not be a symlink.

I'm relatively sure readlink in that fashion is not POSIX - meaning, it won't work on all of bash, zsh, dash, sh, and ksh. PRs welcome if you can convince me that it is!