[tryjobs] Fix result comparison for tryjobs against release branches

Previously, the branch was ignored and tryjobs always compared against
master.

Change-Id: Ifde8dcc4fb4f93fb00fb19e26676f31a790bc64a
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/139760
Reviewed-by: William Hesse <whesse@google.com>
5 files changed