When revision is not specified, currently HEAD is checked out.
Change the default to refs/heads/master

Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Bug: 940214
Change-Id: I8be59095d90447ea898d5247a6433f0734f5e63b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1554862
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>