mktag tests: use "test_commit" helper

Replace ad-hoc setup of a single commit in the "mktag" tests with our
standard helper pattern. The old setup dated back to 446c6faec6 (New
tests and en-passant modifications to mktag., 2006-07-29) before the
helper existed.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3800-mktag.sh

@@ -23,9 +23,7 @@ check_verify_failure () {
 # first create a commit, so we have a valid object/type
 # for the tag.
 test_expect_success 'setup' '
-	echo Hello >A &&
-	git update-index --add A &&
-	git commit -m "Initial commit" &&
+	test_commit A &&
 	head=$(git rev-parse --verify HEAD)
 '