Display the abbreviation for each author in the Author List

[movermeyer]

Display the abbreviation the system uses for each author in the output of git pair:

Perhaps something like:

Current config:
  Email template:
    Author list: Bob McKenzie <bob.mckenzie@example.com> (abbr: BM)
                 Doug McKenzie <doug.mckenzie@example.com> (abbr: DM)

Current author: Arthur Dent
  Current email: arthur.dent@example.com (abbr: AD)

[movermeyer]

If you'd like, I'll submit a PR for this tomorrow.