Set branchref on Changelist if set by user

R=gavinmak@google.com

Bug: 1067814
Change-Id: I592407cd489ade8bffc5158a86ccb061064d3ddd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2815397
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_cl.py

@@ -4257,7 +4257,7 @@ def CMDupload(parser, args):
     # Load default for user, repo, squash=true, in this order.
     options.squash = settings.GetSquashGerritUploads()
 
-  cl = Changelist()
+  cl = Changelist(branchref=options.target_branch)
   # Warm change details cache now to avoid RPCs later, reducing latency for
   # developers.
   if cl.GetIssue():