Return unicode commit hashes in checkout.GitCheckout.

The motivation for this CL is:
When a project is turned on to use GitCheckout with try job verifiers, then the CQ crashes because of line 64 in http://src.chromium.org/viewvc/chrome/trunk/tools/commit-queue/pending_manager.py

This is because revision can only be one of (None, int, unicode). This crash only happens when try jobs are used (which is why I did not see it before).
This CL changes the return type to be unicode instead of string.

Review URL: https://chromiumcodereview.appspot.com/25686005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@226743 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/checkout.py b/checkout.py
index 3d15e50..97a193f 100644
--- a/checkout.py
+++ b/checkout.py
@@ -622,8 +622,8 @@
          '--quiet'])
 
   def _get_head_commit_hash(self):
-    """Gets the current revision from the local branch."""
-    return self._check_output_git(['rev-parse', 'HEAD']).strip()
+    """Gets the current revision (in unicode) from the local branch."""
+    return unicode(self._check_output_git(['rev-parse', 'HEAD']).strip())
 
   def apply_patch(self, patches, post_processors=None, verbose=False):
     """Applies a patch on 'working_branch' and switches to it.