Don't use Git in the mimemagic shim

Git isn't installed in our CNG runtime containers, so when the Gemspec
is loaded an error is produced. Since we don't actually need to use Git,
and only include two files, we can just hardcode the list of files to
include in the Gem.

vendor/shims/mimemagic/mimemagic.gemspec

@@ -12,10 +12,7 @@ Gem::Specification.new do |spec|
   spec.license       = "MIT"
   spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
 
-  # 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(File.expand_path('..', __FILE__)) do
-    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
-  end
+  spec.files = %w[lib/mimemagic.rb lib/mimemagic/version.rb]
+
   spec.require_paths = ["lib"]
 end