git-svn: remove ''--add-author-from' for 'commit-diff'

The subcommand 'commit-diff' does not support the option
'--add-author-from'.

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

Author: asheiduk

Documentation/git-svn.txt

@@ -700,7 +700,7 @@ creating the branch or tag.
 config key: svn.useLogAuthor
 
 --add-author-from::
-	When committing to svn from Git (as part of 'commit-diff', 'set-tree' or 'dcommit'
+	When committing to svn from Git (as part of 'set-tree' or 'dcommit'
 	operations), if the existing log message doesn't already have a
 	`From:` or `Signed-off-by:` line, append a `From:` line based on the
 	Git commit's author string.  If you use this, then `--use-log-author`