Ensure git is executed inside the gemspec dir

When a Gemfile was pointing to a local gem using git to list its files the git command
was executed from within the wrong dir. This fixes the issue without:

1. introducing new dependencies (e.g. `shellwords`)
2. any risks with non-trivial paths (e.g. paths containing spaces)

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

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
       "public gem pushes."
   end
 
-  spec.files         = `git ls-files -z`.split("\x0").reject do |f|
+  spec.files         = Dir.chdir(__dir__){`git ls-files -z`}.split("\x0").reject do |f|
     f.match(%r{^(test|spec|features)/})
   end
   spec.bindir        = "exe"