t7004-tag.sh: re-arrange git tag comment for clarity

t7004-tag.sh: re-arrange git tag comment for clarity

Split the "message in editor has initial comment" test into three
tests. The motivation is to be able to only skip the middle part under
NO_GETTEXT_POISON.

In addition the return value of 'git tag' was being returned. We now
check that it's non-zero. I used ! instead of test_must_fail so that
the GIT_EDITOR variable was only used in this command invocation, and
because the surrounding tests use this style.

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