resolve the default remote branch dynamically

Rather than hardcode or fall over between branches, look up the
default the remote repo is using, and go with that locally too.

BUG=chromium:1126855, b:173131586
TEST=`./run_tests.sh` passes

Change-Id: I2b6b4aeb6ea404e68d32673c807187c8061d9f16
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2693059
Reviewed-by: Julio Hurtado <juahurta@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
4 files changed