git pull is no good after doing rebase upstream

More generally, when you rewrite history in upstream, git will try to merge.
Not only that merging could fail with conflicts, but it also needs git username.

This was my log shown at http://{mydiscourse.com}/admin/docker when doing upgrade:

    $ cd /var/www/discourse && git pull
    *** Please tell me who you are.

    Run

      git config --global user.email "you@example.com"
      git config --global user.name "Your Name"

    to set your account's default identity.
    Omit --global to set the identity only in this repository.

    fatal: empty ident   not allowed