[rubygems/rubygems] Stop publishing Gemfile in default gem template

Similarly to how the other ignored files are intended for local
development and not for production, the Gemfile and Gemfile.lock files
for a gem only relate to local development and aren't useful to people
installing the gem.

https://github.com/rubygems/rubygems/commit/59049c04be

lib/bundler/templates/newgem/newgem.gemspec.tt

@@ -29,7 +29,8 @@ Gem::Specification.new do |spec|
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
   spec.files = Dir.chdir(__dir__) do
     `git ls-files -z`.split("\x0").reject do |f|
-      (File.expand_path(f) == __FILE__) || f.start_with?(*%w[bin/ test/ spec/ features/ .git .circleci appveyor])
+      (File.expand_path(f) == __FILE__) ||
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git .circleci appveyor Gemfile])
     end
   end
   spec.bindir = "exe"

spec/bundler/commands/newgem_spec.rb

@@ -638,6 +638,14 @@ RSpec.describe "bundle gem" do
       expect(bundler_gemspec.files).not_to include("#{gem_name}.gemspec")
     end
 
+    it "does not include the Gemfile file in files" do
+      bundle "gem #{gem_name}"
+
+      bundler_gemspec = Bundler::GemHelper.new(bundled_app(gem_name), gem_name).gemspec
+
+      expect(bundler_gemspec.files).not_to include("Gemfile")
+    end
+
     it "runs rake without problems" do
       bundle "gem #{gem_name}"