Add rbenv help --usage

libexec/rbenv-help

@@ -110,9 +110,23 @@ print_help() {
   fi
 }
 
-if [ -z "$1" ]; then
-  rbenv---version
+print_usage() {
+  local command="$1"
+  local summary usage help
+  eval "$(documentation_for "$command")"
+  [ -z "$usage" ] || echo "$usage"
+}
+
+unset usage
+if [ "$1" = "--usage" ]; then
+  usage="1"
+  shift
+fi
+
+if [ -z "$1" ] || [ "$1" == "rbenv" ]; then
+  [ -n "$usage" ] || rbenv---version
   echo "Usage: rbenv <command> [<args>]"
+  [ -z "$usage" ] || exit
   echo
   echo "Some useful rbenv commands are:"
   print_summaries commands rehash global local shell version versions which whence
@@ -122,7 +136,11 @@ if [ -z "$1" ]; then
 else
   command="$1"
   if [ -n "$(command_path "$command")" ]; then
-    print_help "$command"
+    if [ -n "$usage" ]; then
+      print_usage "$command"
+    else
+      print_help "$command"
+    fi
   else
     echo "rbenv: no such command \`$command'" >&2
     exit 1