Add map_user() and clear_mailmap() to mailmap

Add map_user() and clear_mailmap() to mailmap

map_user() allows to lookup and replace both email and
name of a user, based on a new style mailmap file.

The possible mailmap definitions are now:

  proper_name <commit_email>                             # Old style
  <proper_email> <commit_email>                          # New style
  proper_name <proper_email> <commit_email>              # New style
  proper_name <proper_email> commit_name <commit_email>  # New style

map_email() operates the same as before, with the
exception that it also will to try to match on a name
passed in through the name return buffer.

clear_mailmap() is needed to now clear the more complex
mailmap structure.

Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>