mktag tests: don't create "mytag" twice

mktag tests: don't create "mytag" twice

Change a test added in e0aaf781f6 (mktag.c: improve verification of
tagger field and tests, 2008-03-27) to not create "mytag", which
should only be created and verified at the end in an earlier test
added in 446c6faec6 (New tests and en-passant modifications to mktag.,
2006-07-29).

While we're at it let's prevent a similar logic error from creeping
into the test by asserting that "mytag" doesn't exist before we create
it. Let's do this by moving the test to use "update-ref", instead of
our own homebrew ad-hoc refstore update.

We're not really testing for anything yet by creating the tag at the
end here. A subsequent commit will change that.

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