Use IO.popen to list files

- Redirect `git ls-files` without shelling out.

- When building by `gem`, `__FILE__` is the path name given in the
  command line, or the gemspec file name in the current directory.  In
  that case, comparison it and expanded path never equal.  Compare
  listed file names with the base name of `__FILE__` instead.

tmpdir.gemspec

@@ -15,9 +15,10 @@ Gem::Specification.new do |spec|
 
   # Specify which files should be added to the gem when it is released.
   # 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 2>#{IO::NULL}`.split("\x0").reject do |f|
-      (File.expand_path(f) == __FILE__) ||
+  gemspec = File.basename(__FILE__)
+  spec.files = IO.popen(%w[git ls-files -z], chdir: __dir__, err: IO::NULL) do |ls|
+    ls.readlines("\x0", chomp: true).reject do |f|
+      (f == gemspec) ||
         f.start_with?(*%w[bin/ test/ spec/ features/ .git Gemfile])
     end
   end