add --help to subcommand completions

ensure subcommand exists, then include --help in its completion output

test/completions.bats

@@ -13,7 +13,7 @@ create_command() {
   create_command "rbenv-hello" "#!$BASH
     echo hello"
   run rbenv-completions hello
-  assert_success ""
+  assert_success "--help"
 }
 
 @test "command with completion support" {
@@ -25,7 +25,11 @@ else
   exit 1
 fi"
   run rbenv-completions hello
-  assert_success "hello"
+  assert_success
+  assert_output <<OUT
+--help
+hello
+OUT
 }
 
 @test "forwards extra arguments" {
@@ -40,6 +44,7 @@ fi"
   run rbenv-completions hello happy world
   assert_success
   assert_output <<OUT
+--help
 happy
 world
 OUT

test/exec.bats

@@ -35,6 +35,7 @@ create_executable() {
   run rbenv-completions exec
   assert_success
   assert_output <<OUT
+--help
 rake
 ruby
 OUT