git new-branch --upstream_current <branch_name>
can create a new branch tracking the current branch.
I need this in my recipe so adding this flag to the
recipe module.

Bug: 1238169
Change-Id: I1816b8d78867848e7a8f80337a6d82db8b6ef777
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3095037
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>