git cl: cache also LABELS data on a change.

This removes 1 RPC when tbr-ing second or later patchset.

Since tests cover TBR only in first upload, there are
no test changes. Since adding test is real PITA here,
keep them as is.

R=ehmaldonado

Change-Id: I90801b06727ce2131a703d6cd6c24ffad798bc8a
Reviewed-on: https://chromium-review.googlesource.com/1242212Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cl.py

@@ -2452,7 +2452,7 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
     # Warm change details cache now to avoid RPCs later, reducing latency for
     # developers.
     self._GetChangeDetail(
-        ['DETAILED_ACCOUNTS', 'CURRENT_REVISION', 'CURRENT_COMMIT'])
+        ['DETAILED_ACCOUNTS', 'CURRENT_REVISION', 'CURRENT_COMMIT', 'LABELS'])
 
     status = self._GetChangeDetail()['status']
     if status in ('MERGED', 'ABANDONED'):

tests/git_cl_test.py

@@ -1088,7 +1088,8 @@ class TestGitCl(TestCase):
       calls += [
         (('GetChangeDetail', 'chromium-review.googlesource.com',
           'my%2Frepo~123456',
-          ['DETAILED_ACCOUNTS', 'CURRENT_REVISION', 'CURRENT_COMMIT']),
+          ['DETAILED_ACCOUNTS', 'CURRENT_REVISION', 'CURRENT_COMMIT', 'LABELS']
+         ),
          {
            'owner': {'email': (other_cl_owner or 'owner@example.com')},
            'change_id': '123456789',