Introduce different repo types (git and gitiles)

recipe_engine/remote_run.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from __future__ import print_function
 
 import contextlib
 import logging
 import os
 import shutil
@@ skipping 18 matching lines @@
   finally:
     if workdir_tempdir:
       shutil.rmtree(args.workdir, ignore_errors=True)
 
 
 def main(args):
   with ensure_workdir(args):
     checkout_dir = os.path.join(args.workdir, 'checkout')
     if args.use_gitiles:
       args.revision = args.revision or 'HEAD'
-      fetch.ensure_gitiles_checkout(
-          args.repository, args.revision, checkout_dir, allow_fetch=True)
+      backend = fetch.GitilesBackend()
     else:
       args.revision = args.revision or 'FETCH_HEAD'
-      fetch.ensure_git_checkout(
-          args.repository, args.revision, checkout_dir, allow_fetch=True)
+      backend = fetch.GitBackend()
+    backend.checkout(
+        args.repository, args.revision, checkout_dir, allow_fetch=True)
     recipes_cfg = package.ProtoFile(
         package.InfraRepoConfig().to_recipes_cfg(checkout_dir))
     cmd = [
         sys.executable,
         os.path.join(
             checkout_dir,
             recipes_cfg.read().recipes_path,
             'recipes.py'),
         'run'
     ] + args.run_args
     logging.info('Running %r', cmd)
     return subprocess.call(cmd)