Merge branch 'sa/cat-file-mailmap'

"git cat-file" learned an option to use the mailmap when showing
commit and tag objects.

* sa/cat-file-mailmap:
  cat-file: add mailmap support
  ident: rename commit_rewrite_person() to apply_mailmap_to_header()
  ident: move commit_rewrite_person() to ident.c
  revision: improve commit_rewrite_person()
