submodule tests: use symbolic-ref --short to discover branch name

Change a use of $GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME added in
704fed9ea2 (tests: start moving to a different default main branch
name, 2020-10-23) to simply discover the initial branch name of a
repository set up in this function with "symbolic-ref --short".

That's something done in another test in 704fed9ea2, so doing it like
this seems like an omission, or rather an overly eager
search/replacement instead of fixing the test logic.

There are only three uses of the GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
variable in the test suite, this gets rid of one of those.

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

t/lib-submodule-update.sh

@@ -63,6 +63,7 @@ create_lib_submodule_repo () {
 	git init submodule_update_repo &&
 	(
 		cd submodule_update_repo &&
+		branch=$(git symbolic-ref --short HEAD) &&
 		echo "expect" >>.gitignore &&
 		echo "actual" >>.gitignore &&
 		echo "x" >file1 &&
@@ -144,7 +145,7 @@ create_lib_submodule_repo () {
 		git checkout -b valid_sub1 &&
 		git revert HEAD &&
 
-		git checkout "${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME-master}"
+		git checkout "$branch"
 	)
 }