bot_update: Fix syncing to branches.

When syncing remote branches, checkout origin/<branch> instead of
<branch>, since there might be a local branch with that name.

Bug: 859703
Change-Id: I9a49b940f53f75a97e5458fdcf578c3b52b26de2
Reviewed-on: https://chromium-review.googlesource.com/1128271
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -553,7 +553,9 @@ def _get_target_branch_and_revision(solution_name, git_url, revisions):
   if len(parts) == 2:
     # Support for "branch:revision" syntax.
     return parts
-  return 'master', configured
+  if COMMIT_HASH_RE.match(configured):
+    return 'master', configured
+  return configured, 'HEAD'
 
 
 def get_target_pin(solution_name, git_url, revisions):