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 <anthonywang513@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9811-git-p4-label-import.sh

@@ -95,9 +95,9 @@ test_expect_success 'two labels on the same changelist' '
 		cd "$git" &&
 		git p4 sync --import-labels &&
 
-		git tag | grep TAG_F1 &&
-		git tag | grep -q TAG_F1_1 &&
-		git tag | grep -q TAG_F1_2 &&
+		git show-ref --verify refs/tags/TAG_F1_1 &&
+		git show-ref --verify refs/tags/TAG_F1_2 &&
+		test_must_fail git show-ref --verify refs/tags/TAG_F1_ONLY &&
 
 		cd main &&
 
@@ -207,8 +207,7 @@ test_expect_success 'use git config to enable import/export of tags' '
 		git tag CFG_A_GIT_TAG &&
 		git p4 rebase --verbose &&
 		git p4 submit --verbose &&
-		git tag &&
-		git tag | grep TAG_F1_1
+		git show-ref --verify refs/tags/TAG_F1_1 &&
 	) &&
 	(
 		cd "$cli" &&