Teach commands in the "log" family to optionally pay attention to
the mailmap.

* ap/log-mailmap:
  log --use-mailmap: optimize for cases without --author/--committer search
  log: add log.mailmap configuration option
  log: grep author/committer using mailmap
  test: add test for --use-mailmap option
  log: add --use-mailmap option
  pretty: use mailmap to display username and email
  mailmap: add mailmap structure to rev_info and pp
  mailmap: simplify map_user() interface
  mailmap: remove email copy and length limitation
  Use split_ident_line to parse author and committer
  string-list: allow case-insensitive string list