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>