Tweak sanitizing shell name

Handles situation when the output is `/bin/shell- args...`. First strip
away the arguments, then the trailing dash.

libexec/rbenv-init

@@ -34,8 +34,8 @@ done
 shell="$1"
 if [ -z "$shell" ]; then
   shell="$(ps -p "$PPID" -o 'args=' 2>/dev/null || true)"
-  shell="${shell##-}"
   shell="${shell%% *}"
+  shell="${shell##-}"
   shell="${shell:-$SHELL}"
   shell="${shell##*/}"
 fi