show-branch tests: rename the one "show-branch" test file

show-branch tests: rename the one "show-branch" test file

Rename the only *show-branch* test file to indicate that more tests
belong it in than just the one-off octopus test it now contains.

The test was initially added in ce567d1867a (Add test to show that
show-branch misses out the 8th column, 2008-07-23) and
11ee57bc4c4 (sort_in_topological_order(): avoid setting a commit flag,
2008-07-23). Those two add almost the same content, one with a
test_expect_success and the other a test_expect_failure (a bug being
tested for was fixed on one of the branches).

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