Merge bundler to standard libraries.

rubygems 2.7.x depends bundler-1.15.x. This is preparation for
  rubygems and bundler migration.

  * lib/bundler.rb, lib/bundler/*: files of bundler-1.15.4
  * spec/bundler/*: rspec examples of bundler-1.15.4. I applied patches.
    * https://github.com/bundler/bundler/pull/6007
    * Exclude not working examples on ruby repository.
    * Fake ruby interpriter instead of installed ruby.
  * Makefile.in: Added test task named `test-bundler`. This task is only
    working macOS/linux yet. I'm going to support Windows environment later.
  * tool/sync_default_gems.rb: Added sync task for bundler.

  [Feature #12733][ruby-core:77172]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59779 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/bundler/plugin/installer/git.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module Bundler
+  module Plugin
+    class Installer
+      class Git < Bundler::Source::Git
+        def cache_path
+          @cache_path ||= begin
+            git_scope = "#{base_name}-#{uri_hash}"
+
+            Plugin.cache.join("bundler", "git", git_scope)
+          end
+        end
+
+        def install_path
+          @install_path ||= begin
+            git_scope = "#{base_name}-#{shortref_for_path(revision)}"
+
+            Plugin.root.join("bundler", "gems", git_scope)
+          end
+        end
+
+        def version_message(spec)
+          "#{spec.name} #{spec.version}"
+        end
+
+        def root
+          Plugin.root
+        end
+
+        def generate_bin(spec, disable_extensions = false)
+          # Need to find a way without code duplication
+          # For now, we can ignore this
+        end
+      end
+    end
+  end
+end