git-svn: allow empty email-address using authors-prog and authors-file

git-svn: allow empty email-address using authors-prog and authors-file

The email address in --authors-file and --authors-prog can be empty but
git-svn translated it into a fictional email address in the form

	jondoe <jondoe@6aafaa21e0fb4338a68ab372a049893d>

containing the SVN repository UUID. Now git-svn behaves like git-commit:
If the email is *explicitly* set to the empty string using '<>', the
commit does not contain an email address, only the name:

	jondoe <>

Allowing to remove the email address *intentionally* prevents automatic
systems from sending emails to those fictional addresses and avoids
cluttering the log output with unnecessary stuff.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Eric Wong <e@80x24.org>