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>

Documentation/git-svn.txt

@@ -635,7 +635,8 @@ config key: svn.findcopiesharder
 
 -A<filename>::
 --authors-file=<filename>::
-	Syntax is compatible with the file used by 'git cvsimport':
+	Syntax is compatible with the file used by 'git cvsimport' but
+	an empty email address can be supplied with '<>':
 +
 ------------------------------------------------------------------------
 	loginname = Joe User <user@example.com>
@@ -654,8 +655,9 @@ config key: svn.authorsfile
 	If this option is specified, for each SVN committer name that
 	does not exist in the authors file, the given file is executed
 	with the committer name as the first argument.  The program is
-	expected to return a single line of the form "Name <email>",
-	which will be treated as if included in the authors file.
+	expected to return a single line of the form "Name <email>" or
+	"Name <>", which will be treated as if included in the authors
+	file.
 +
 Due to historical reasons a relative 'filename' is first searched
 relative to the current directory for 'init' and 'clone' and relative