git-svn: stop repeatedly reusing the first commit message with dcommit

Excessive use of global variables got me into trouble.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/git-svn.perl b/git-svn.perl
index 9382a15..0290850 100755
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -819,6 +819,7 @@
 	} else {
 		$ed->close_edit;
 	}
+	$_message = $_file = undef;
 }
 
 ########################### utility functions #########################