Add hello-world Go executable and gitaly-proto 0.5.0

Add basic plumbing to have Go executables in gitlab-shell. We want to use this for Gitaly integration.

I have already included gitaly-proto via govendor even though it is not used yet.

Previous discussion in https://gitlab.com/gitlab-org/gitlab-shell/issues/80

Closes https://gitlab.com/gitlab-org/gitaly/issues/186

bin/compile

+#!/usr/bin/env ruby
+
+require 'fileutils'
+
+# This will cause ROOT_PATH to get set
+require_relative '../lib/gitlab_init'
+
+GO_DIR = 'go'
+GOPATH = File.join(ROOT_PATH, 'go_build')
+GO_PACKAGE = File.join('gitlab.com/gitlab-org/gitlab-shell', GO_DIR)
+
+def main
+  FileUtils.rm_rf(GOPATH)
+  build_source_dir = File.join(GOPATH, 'src', GO_PACKAGE)
+  FileUtils.mkdir_p(build_source_dir)
+  FileUtils.cp_r(File.join(ROOT_PATH, GO_DIR, '.'), build_source_dir)
+  run!({ 'GOPATH' => GOPATH }, %W[go install #{GO_PACKAGE}/cmd/...])
+  executables = Dir["#{File.join(GOPATH, 'bin')}/*"]
+  FileUtils.chmod(0755, executables)
+  FileUtils.cp(executables, File.join(ROOT_PATH, 'bin'))
+end
+
+def run!(env, cmd)
+  raise "env must be a hash" unless env.is_a?(Hash)
+  raise "cmd must be an array" unless cmd.is_a?(Array)
+
+  if !system(env, *cmd)
+    abort "command failed: #{env.inspect} #{cmd.join(' ')}"
+  end
+end
+
+main