Save username -> Full Name <email@addr.es> map file

Save username -> Full Name <email@addr.es> map file

When the user specifies a username -> Full Name <email@addr.es> map
file with the -A option, save a copy of that file as
$git_dir/svn-authors. When running git-svnimport with an existing GIT
directory, use $git_dir/svn-authors (if it exists) unless a file was
explicitly specified with -A.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>