contrib: update stats/mailmap script

contrib: update stats/mailmap script

This version changes quite a few things:

  1. The original parsed the mailmap file itself, and it did
     it wrong (it did not understand entries with an extra
     email key).

     Instead, this version uses git's "%aE" and "%aN"
     formats to have git perform the mapping, meaning we do
     not have to read .mailmap at all, but still operate on
     the current state that git sees (and it also works
     properly from subdirs).

  2. The original would find multiple names for an email,
     but not the other way around.

     This version can do either or both. If we find multiple
     emails for a name, the resolution is less obvious than
     the other way around. However, it can still be a
     starting point for a human to investigate.

  3. The original would order only by count, not by recency.

     This version can do either. Combined with showing the
     counts, it can be easier to decide how to resolve.

  4. This version shows similar entries in a blank-delimited
     stanza, which makes it more clear which options you are
     picking from.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>