Add ability to mock git ref

R=nodir@chromium.org

Bug: 903688
Change-Id: I03d3c8c35059e9d7ade60d6236a9228deb6ce967
Reviewed-on: https://chromium-review.googlesource.com/c/1328644
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
diff --git a/recipe_modules/buildbucket/test_api.py b/recipe_modules/buildbucket/test_api.py
index 4e478f0..9d0bb4e 100644
--- a/recipe_modules/buildbucket/test_api.py
+++ b/recipe_modules/buildbucket/test_api.py
@@ -42,6 +42,7 @@
       bucket='ci',  # shortname.
       builder='builder',
       git_repo=None,
+      git_ref='refs/heads/master',
       revision='2d72510e447ab60a9728aeea2362d8be2cbd7789',
       build_number=0):
     """Returns a typical buildbucket CI build scheduled by luci-scheduler."""
@@ -64,7 +65,7 @@
             gitiles_commit=common_pb2.GitilesCommit(
                 host=gitiles_host,
                 project=gitiles_project,
-                ref='refs/heads/master',
+                ref=git_ref,
                 id=revision,
             ),
         ),