git cl status: re-use existing ChangeList object.

This way we avoid extra RPC to codereview site.

R=sergiyb@chromium.org
BUG=681704

Change-Id: I81f9ac1b8fe4cfaa95458a5669735168185c47e5
Reviewed-on: https://chromium-review.googlesource.com/430639
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>

git_cl.py

@@ -3821,10 +3821,13 @@ def CMDstatus(parser, args):
           alignment, ShortBranchName(branch), color, url,
           status_str, reset))
 
-  cl = Changelist(auth_config=auth_config)
+
+  branch = GetCurrentBranch()
   print()
-  print('Current branch:',)
-  print(cl.GetBranch())
+  print('Current branch: %s' % branch)
+  for cl in changes:
+    if cl.GetBranch() == branch:
+      break
   if not cl.GetIssue():
     print('No issue assigned.')
     return 0