Add repo start option to create the branch based off HEAD

This makes it way easier to recover from forgetting to run repo start
before committing: just run `repo start -b new-branch`, instead of
all that tedious mucking around with reflogs.

Change-Id: I56d49dce5d027e28fbba0507ac10cd763ccfc36d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/232712
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>