tests: set GIT_CEILING_DIRECTORIES to prevent repo discovery

When running `npm test` from a Git worktree, test-spawned git processes
that happened to operate at or above `.test-dir/` would walk up the
directory tree and discover the enclosing real repository. In a worktree
setup this is particularly dangerous because the `.git/config` is shared
across all worktrees, so any accidental config write (e.g. `core.bare`)
affects every checkout.

Prevent this by setting `GIT_CEILING_DIRECTORIES` to every ancestor of
the `.test-dir/` temporary directory. The temp dir itself is excluded so
that git can still discover the test repositories created inside it.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

tests/test-lib.ts

@@ -141,6 +141,13 @@ export async function testCreateRepo(name: string, suffix?: string): Promise<Tes
     delete process.env.GIT_DIR;
     delete process.env.GIT_WORK_TREE;
 
+    // Prevent git from discovering the enclosing repository
+    const ceilings = [];
+    for (let dir = tmp, parent; (parent = path.dirname(dir)) !== dir; dir = parent) {
+        ceilings.push(parent);
+    }
+    process.env.GIT_CEILING_DIRECTORIES = ceilings.join(path.delimiter);
+
     // eslint-disable-next-line security/detect-unsafe-regex
     const match = name.match(/^(.*[\\/])?(.*?)(\.test)?\.ts$/);
     if (match) {