Pass git_repo param when calling buildbucket.test_api.try_build.

TBR=hinoka@chromium.org
Change-Id: Ic4a3d270ef3477cb21e4e02d8a22caf7996757b3
Reviewed-on: https://chromium-review.googlesource.com/1213555Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>