git-svn: Set svn.authorsfile to an absolute path when cloning

If --authors-file is passed a relative path, cloning will work, but
future `git svn fetch`es will fail to locate the authors file
correctly.  Thus, use File::Spec->rel2abs to determine an absolute
path for the authors file before setting it.

Signed-off-by: Alex Vandiver <alex@chmrr.net>
Acked-by: Eric Wong <normalperson@yhbt.net>