Add root_dir= to shell completion

priv/shell-completion/bash/rebar

@@ -12,7 +12,7 @@ _rebar()
         ct doc delete-deps eunit get-deps generate generate-upgrade \
         help list-deps list-templates update-deps version xref overlay \
         case= force=1 jobs= suite= verbose=1 appid= previous_release= \
-        skip_deps=true skip_app= template= template_dir="
+        root_dir= skip_deps=true skip_app= template= template_dir="
 
     if [[ ${cur} == --* ]] ; then
         COMPREPLY=( $(compgen -W "${lopts}" -- ${cur}) )