t7300: add testcase showing unnecessary traversal into ignored directory

t7300: add testcase showing unnecessary traversal into ignored directory

The PNPM package manager is apparently creating deeply nested (but
ignored) directory structures; traversing them is costly
performance-wise, unnecessary, and in some cases is even throwing
warnings/errors because the paths are too long to handle on various
platforms.  Add a testcase that checks for such unnecessary directory
traversal.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>