Use git_cl.Changelist.GetUpstreamBranch() for diff in submit_try

BUG=skia:1862
R=epoger@google.com

Review URL: https://codereview.chromium.org/136823002

git-svn-id: http://skia.googlecode.com/svn/trunk@13066 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed