t7004: check existence of correct tag

We try to delete the non-existing tag "anothertag", but for the verifications, we check that the tag "myhead" doesn't exist. "myhead" isn't used in this test except for this checking. Comparing to the test two tests earlier, it looks like a copy-paste mistake. Perhaps it's overkill to check that `git tag -d` didn't decide to *create* a tag. But since we're trying to be this careful, let's actually check the correct tag. While we're doing this, let's use a more descriptive tag name instead -- "nonexistingtag" should be obvious. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>