reflog tests: add --updateref tests

reflog tests: add --updateref tests

Add tests that cover blindspots in "git reflog delete --updateref"
behavior. Before this change removing the "type & REF_ISSYMREF" check
added in 5e6f003ca8a (reflog_expire(): ignore --updateref for symbolic
references, 2015-03-03) would not fail any tests.

The "--updateref" option was added in 55f10565371 (git-reflog: add
option --updateref to write the last reflog sha1 into the ref,
2008-02-22) for use in git-stash.sh, see e25d5f9c82e (git-stash: add
new 'drop' subcommand, 2008-02-22).

Even though the regression test I need is just the "C" case here,
let's test all these combinations for good measure. I started out
doing these as a for-loop, but I think this is more readable.

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