test libs: rename bundle helper to "lib-bundle.sh"

Rename the recently introduced test-bundle-functions.sh to be
consistent with other lib-*.sh files, which is the convention for
these sorts of shared test library functions.

The new test-bundle-functions.sh was introduced in 9901164d81d (test:
add helper functions for git-bundle, 2021-01-11). It was the only
test-*.sh of this nature.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 42f5503..c1e0eb3 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -9,7 +9,7 @@
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
-. "$TEST_DIRECTORY"/test-bundle-functions.sh
+. "$TEST_DIRECTORY"/lib-bundle.sh
 
 D=$(pwd)