t7030: stop using invalid tag name

t7030: stop using invalid tag name

We intentionally invalidate the signature of a tag by switching its tag
name from "seventh" to "7th forged". However, the latter is not a valid
tag name because it contains a space. This doesn't currently affect the
test, but we're better off using something syntactically valid. That
reduces the number of possible failure modes in the test, and
future-proofs us if git hash-object gets more picky about its input.

The t7031 script, which was mostly copied from t7030, has the same
problem, so we'll fix it, too.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>