rebase tests: test linear branch topology

rebase tests: test linear branch topology

Add tests rebasing a linear branch topology to linear rebase tests
added in 2aad7cace2 ("add simple tests of consistency across rebase
types", 2013-06-06).

These tests are duplicates of two surrounding tests that do the same
with tags pointing to the same objects. Right now there's no change in
behavior being introduced, but as we'll see in a subsequent change
rebase can have different behaviors when working implicitly with
remote tracking branches.

While I'm at it add a --fork-point test, strictly speaking this is
redundant to the existing '' test, as no argument to rebase implies
--fork-point. But now it's easier to grep for tests that explicitly
stress --fork-point.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>