t7510: stop referring to master in later tests

t7510: stop referring to master in later tests

Our setup creates a sequence of commits, each with its own
tag. However, we sometimes refer to "seventh-signed" as
"master". This works, since it is at the tip of the created
branch, but is brittle if new tests need to add more
commits. Let's use its tag name to be unambiguous.

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