[rubygems/rubygems] Fix bundle plugin install --help showing bundle install's help

https://github.com/rubygems/rubygems/commit/b7b7d16aa8

lib/bundler/cli.rb

@@ -14,6 +14,7 @@ module Bundler
     COMMAND_ALIASES = {
       "check" => "c",
       "install" => "i",
+      "plugin" => "",
       "list" => "ls",
       "exec" => ["e", "ex", "exe"],
       "cache" => ["package", "pack"],

spec/bundler/plugins/install_spec.rb

@@ -29,6 +29,12 @@ RSpec.describe "bundler plugin install" do
     plugin_should_be_installed("foo")
   end
 
+  it "shows help when --help flag is given" do
+    bundle "plugin install --help"
+
+    expect(out).to include("bundle plugin install PLUGINS    # Install the plugin from the source")
+  end
+
   context "plugin is already installed" do
     before do
       bundle "plugin install foo --source #{file_uri_for(gem_repo2)}"