This fixes:

    fatal: Cannot setup tracking information; starting point 'BRANCH' is not a branch.

if the remote refspec is set up to only fetch specific branches and NOT
like the default `+refs/heads/*:refs/remotes/origin/*`.

I don't understand why would git, after fetching into
`refs/remotes/origin/BRANCH`, complain that `origin/BRANCH` is not a
branch, but there we have it. Setting up upstream tracking manually
works around this problem.