Expand literal tilde in PATH

The literal tilde in a PATH entry (e.g. `~/.rbenv/shims`) doesn't seem
to be supported by system `which` utility, but *does* seem to be
supported by `command -v` (used in `rbenv-which`) and `type -p`.

Therefore, we must strip away `~/.rbenv/shims` from PATH when looking up
executables for system Ruby, lest we risk infinite loop. We do so by
substituting any occurence of `~` in PATH with the value of `HOME`.

libexec/rbenv-which

@@ -18,7 +18,7 @@ fi
 remove_from_path() {
   local path_to_remove="$1"
   local path_before
-  local result=":$PATH:"
+  local result=":${PATH//\~/$HOME}:"
   while [ "$path_before" != "$result" ]; do
     path_before="$result"
     result="${result//:$path_to_remove:/:}"