Set an artificial $HOME for gitaly in test

scripts/gitaly-test-spawn

 #!/usr/bin/env ruby
 gitaly_dir = 'tmp/tests/gitaly'
+env = { 'HOME' => File.expand_path('tmp/tests') }
 args = %W[#{gitaly_dir}/gitaly #{gitaly_dir}/config.toml]
 # Print the PID of the spawned process
-puts spawn(*args, [:out, :err] => 'log/gitaly-test.log')
+puts spawn(env, *args, [:out, :err] => 'log/gitaly-test.log')

spec/spec_helper.rb

@@ -207,3 +207,6 @@ Shoulda::Matchers.configure do |config|
     with.library :rails
   end
 end
+
+# Prevent Rugged from picking up local developer gitconfig.
+Rugged::Settings['search_path_global'] = Rails.root.join('tmp/tests').to_s