Deduplicate shims in $PATH for the fish shell during initialization (#430)

When a fish login shell is invoked within an existing login shell, avoid
duplicating path to the shims by removing existing occurrences before
prepending.

bin/pyenv-virtualenv-init

@@ -86,6 +86,8 @@ fi
 case "$shell" in
 fish )
   cat <<EOS
+while set index (contains -i -- "${PYENV_VIRTUALENV_ROOT:-${PYENV_VIRTUALENV_INSTALL_PREFIX}}/shims" \$PATH)
+set -eg PATH[\$index]; end; set -e index
 set -gx PATH '${PYENV_VIRTUALENV_ROOT:-${PYENV_VIRTUALENV_INSTALL_PREFIX}}/shims' \$PATH;
 set -gx PYENV_VIRTUALENV_INIT 1;
 EOS