Fix symlinks to '..' to fix rubygems improperly expanding symlinked
paths. Without this fix, some gems fail to install. This patch is applied in
rubygems 2.5.2, but ruby version 2.3.1 bundles an older version of rubygems
(2.5.1).

--- a/lib/rubygems/package.rb
+++ b/lib/rubygems/package.rb
@@ -383,7 +383,7 @@ def extract_tar_gz io, destination_dir, pattern = "*" # :nodoc:
           FileUtils.chmod entry.header.mode, destination
         end if entry.file?

-        File.symlink(install_location(entry.header.linkname, destination_dir), destination) if entry.symlink?
+        File.symlink(entry.header.linkname, destination) if entry.symlink?

         verbose destination
       end
diff --git a/test/rubygems/test_gem_package.rb b/test/rubygems/test_gem_package.rb
index 7848bc2..f287bd3 100644
--- a/test/rubygems/test_gem_package.rb
+++ b/test/rubygems/test_gem_package.rb
@@ -428,19 +428,25 @@ def test_extract_tar_gz_absolute
                  "#{@destination} is not allowed", e.message)
   end

-  def test_extract_tar_gz_symlink_absolute
+  def test_extract_tar_gz_symlink_relative_path
+    skip 'symlink not supported' if Gem.win_platform?
+
     package = Gem::Package.new @gem

     tgz_io = util_tar_gz do |tar|
-      tar.add_symlink 'code.rb', '/absolute.rb', 0644
+      tar.add_file    'relative.rb', 0644 do |io| io.write 'hi' end
+      tar.mkdir       'lib',         0755
+      tar.add_symlink 'lib/foo.rb', '../relative.rb', 0644
     end

-    e = assert_raises Gem::Package::PathError do
-      package.extract_tar_gz tgz_io, @destination
-    end
+    package.extract_tar_gz tgz_io, @destination

-    assert_equal("installing into parent path /absolute.rb of " +
-                 "#{@destination} is not allowed", e.message)
+    extracted = File.join @destination, 'lib/foo.rb'
+    assert_path_exists extracted
+    assert_equal '../relative.rb',
+                 File.readlink(extracted)
+    assert_equal 'hi',
+                 File.read(extracted)
   end

   def test_extract_tar_gz_directory