show-branch tests: add missing tests

show-branch tests: add missing tests

Add missing tests for --remotes, --list and --merge-base. These are
not exhaustive, but better than the nothing we have now.

There were some tests for this command added in f76412ed6db ([PATCH]
Add 'git show-branch'., 2005-08-21) has never been properly tested,
namely for the --all option in t6432-merge-recursive-space-options.sh,
and some of --merge-base and --independent in t6010-merge-base.sh.

This fixes a few more blind spots, but there's still a lot of behavior
that's not tested for.

These new tests show the odd (and possibly unintentional) behavior of
--merge-base with one argument, and how its output is the same as "git
merge-base" with N bases in this particular case. See the test added
in f621a8454d1 (git-merge-base/git-show-branch --merge-base:
Documentation and test, 2009-08-05) for a case where the two aren't
the same.

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