t5619: wrap test_commit_bulk in GIT_DIR subshell for bare repo

To prepare for `safe.bareRepository` defaulting to `explicit` (see
8d1a744), wrap the `test_commit_bulk` call in `(GIT_DIR="$REPO" &&
export GIT_DIR && test_commit_bulk ...)` because `test_commit_bulk -C`
relies on implicit discovery which would fail once the default changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t5619-clone-local-ambiguous-transport.sh

@@ -21,7 +21,7 @@ test_expect_success 'setup' '
 	echo "secret" >sensitive/secret &&
 
 	git init --bare "$REPO" &&
-	test_commit_bulk -C "$REPO" --ref=main 1 &&
+	(GIT_DIR="$REPO" && export GIT_DIR && test_commit_bulk --ref=main 1) &&
 
 	git -C "$REPO" update-ref HEAD main &&
 	git -C "$REPO" update-server-info &&