Let git-svnimport's author file use same syntax as git-cvsimport's

Let git-svnimport's author file use same syntax as git-cvsimport's

git-cvsimport uses a username => Full Name <email@addr.es> mapping
file with this syntax:

  kha=Karl Hasselström <kha@treskal.com>

Since there is no reason to use another format for git-svnimport, use
the same format.

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