git-svn: Add --add-author-from option.

git-svn: Add --add-author-from option.

This option adds a From: line (based on the commit's author information)
at the beginning of the body of the commit log message when sending to
svn, if a From: or Signed-off-by: header does not exist.

This, combined with --use-log-author, can retain the author field of commits
through a round trip from git to svn and back.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>