contrib: script to create a new vendor branch

This script is intended to create a new vendor branch.  Doing so is
not completely obvious if you are not familiar with the upstream
structure, so this takes the pain out of getting it right.

It doesn't check out the branch locally, but does set everything up so
that, if you have push enabled for your vendor branches, then

  git push vendors/<vendor> <branch>

will work as expected.

Run the script as

  contrib/git-add-vendor-branch.sh <vendor>/<branch> <start-point>

the <vendor> space must have previously been set up in the way
git-fetch-vendor.sh expects.

    * git-add-vendor-branch.sh: New file.