t3418: decouple test cases from a previous `rebase -p` test case

It is in general a good idea for regression test cases to be as
independent of each other as possible (with the one exception of an
initial `setup` test case, which is only a test case in Git's test suite
because it does not have a notion of a fixture or setup).

This patch addresses one particular instance of this principle being
violated: a few test cases in t3418-rebase-continue.sh depend on a side
effect of a test case that verifies a specific `rebase -p` behavior. The
later test cases should, however, still succeed even if the `rebase -p`
test case is skipped.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3418-rebase-continue.sh

@@ -177,6 +177,7 @@ test_expect_success 'setup rerere database' '
 	git checkout master &&
 	test_commit "commit-new-file-F3" F3 3 &&
 	test_config rerere.enabled true &&
+	git update-ref refs/heads/topic commit-new-file-F3-on-topic-branch &&
 	test_must_fail git rebase -m master topic &&
 	echo "Resolved" >F2 &&
 	cp F2 expected-F2 &&