download: add a --branch option

This allows people to quickly create new branches when pulling down
changes rather than having to juggle the git calls themselves.

Bug: https://crbug.com/gerrit/11609
Change-Id: Ie6a4d05e9f4e9347fe7f7119c768e6446563ae65
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259855


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>