Avoid spurious error messages on error mistakes.

Prior to that, if the user chose "squash" as a first action, the stderr
looked like:

    grep: /home/madcoder/dev/scm/git/.git/rebase-merge/done: No such file or directory
    Cannot 'squash' without a previous commit

Now the first line is gone.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -309,7 +309,7 @@ do_next () {
 	squash|s)
 		comment_for_reflog squash
 
-		has_action "$DONE" ||
+		test -f "$DONE" && has_action "$DONE" ||
 			die "Cannot 'squash' without a previous commit"
 
 		mark_action_done