svn: properly escape arguments for authors-prog

Previously, the call to authors-prog was not properly escaped, so any
special characters in the Subversion username, such as spaces and
semi-colons, would be interpreted by the shell rather than being passed
in as the first argument.  Now all unsafe characters are escaped using
"git rev-parse --sq-quote"

[ew: switched from "\Q..\E" to "rev-parse --sq-quote"]

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-svn.perl b/git-svn.perl
index d075810..24bdbf5 100755
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -2810,6 +2810,7 @@
 
 sub call_authors_prog {
 	my ($orig_author) = @_;
+	$orig_author = command_oneline('rev-parse', '--sq-quote', $orig_author);
 	my $author = `$::_authors_prog $orig_author`;
 	if ($? != 0) {
 		die "$::_authors_prog failed with exit code $?\n"