t3011: demonstrate directory traversal failures

t3011: demonstrate directory traversal failures

Add several tests demonstrating directory traversal failures of various
sorts in dir.c (and one similar looking test that turns out to be a
git_fnmatch bug).  A lot of these tests look like near duplicates of
each other, but an optimization path in dir.c to pre-descend into a
common prefix and the specialized treatment of trailing slashes in dir.c
mean the tiny differences are sometimes important and potentially cause
different codepaths to be explored.

Of the 7 failing tests, 2 are new to git-2.24.0 (tweaked by side effects
of the en/clean-nested-with-ignored-topic); the other 5 also failed
under git-2.23.0 and earlier.

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