t3406: rework rebase reflog tests

t3406: rework rebase reflog tests

Refactor the tests in preparation for adding more tests in the next
few commits. The reworked tests use the same function for testing both
the "merge" and "apply" backends. The test coverage for the "apply"
backend now includes setting GIT_REFLOG_ACTION.

Note that rebasing the "conflicts" branch does not create any
conflicts yet. A commit to do that will be added in the next commit
and the diff ends up smaller if we have don't rename the branch when
it is added.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>