[PATCH v2] test-lib: set GIT_CEILING_DIRECTORIES to protect the surrounding repository

Every once in a while a test somehow manages to escape from its trash directory and modifies the surrounding repository, whether because of a bug in git itself, a bug in a test [1], or e.g. when trying to run tests with a shell that is, in general, unable to run our tests [2].

Set GIT_CEILING_DIRECTORIES="$TRASH_DIRECTORY/.." as an additional safety measure to protect the surrounding repository at least from modifications by git commands executed in the tests (assuming that handling of ceiling directories during repository discovery is not broken, and, of course, it won't save us from regular shell commands, e.g. 'cd .. && rm -f ...').

[1] e.g. https://public-inbox.org/git/20210423051255.GD2947267@szeder.dev
[2] $ git symbolic-ref HEAD
    refs/heads/master
    $ ksh ./t2011-checkout-invalid-head.sh
    [... a lot of "not ok" ...]
    $ git symbolic-ref HEAD
    refs/heads/other

    (In short: 'ksh' doesn't support the 'local' builtin command,
    which is used by 'test_oid', causing it to return with error
    whenever it's called, leaving ZERO_OID set to empty, so when the
    test 'checkout main from invalid HEAD' runs 'echo $ZERO_OID
    >.git/HEAD' it writes a corrupt (not invalid) HEAD, and subsequent
    git commands don't recognize the repository in the trash directory
    anymore, but operate on the surrounding repo.)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
Changes since v1:
 - Set "$TRASH_DIRECTORY/.." instead of $GIT_BUILD_DIR.
 - Minor commit message tweaks.

 t/test-lib.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)