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

Excessive use of global variables got me into trouble.
Signed-off-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-svn.perl

@@ -819,6 +819,7 @@ sub commit_diff {
 	} else {
 		$ed->close_edit;
 	}
+	$_message = $_file = undef;
 }
 
 ########################### utility functions #########################