t9811: be more precise to check importing of tags

t9811: be more precise to check importing of tags

The tests use grep to search the output of `git tag` for tagnames they
expect to exist, which can incorrectly pass if an unxpected tag
has the expected tag as its substring. We fix this by using `git
show-ref --verify` instead.

Additionally, we add a negative test to verify that a possible
uninteded tag does not show up in the imported repository.

This change also fixes an additional problem, where piping the
output of `git tag` caused the exit codes to be lost.

Signed-off-by: Anthony Wang <redacted>
Signed-off-by: Junio C Hamano <redacted>