bot_update: Rewrite repo@HEAD to repo@origin/master

BUG=692140,664880

Change-Id: I3ddd899f1841930e601a906d658c5c78e2d76673
Reviewed-on: https://chromium-review.googlesource.com/442707Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>

recipe_modules/bot_update/resources/bot_update.py

@@ -354,6 +354,8 @@ def gclient_sync(with_branch_heads, shallow, revisions, break_repo_locks):
   if break_repo_locks:
     args += ['--break_repo_locks']
   for name, revision in sorted(revisions.iteritems()):
+    if revision.upper() == 'HEAD':
+      revision = 'origin/master'
     args.extend(['--revision', '%s@%s' % (name, revision)])
 
   try:

tests/bot_update_coverage_test.py

@@ -215,7 +215,7 @@ class BotUpdateUnittests(unittest.TestCase):
         '--revision', idx_first_revision+1)
     with self.assertRaises(ValueError):
       args.index('--revision', idx_second_revision+1)
-    self.assertEquals(args[idx_first_revision+1], 'src@HEAD')
+    self.assertEquals(args[idx_first_revision+1], 'src@origin/master')
     self.assertEquals(args[idx_second_revision+1], 'src/v8@deadbeef')
     return self.call.records