[PATCH] 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="$GIT_BUILD_DIR" as an additional safety measure to protect the surrounding repository at least from modifications by git commands executed in the tests (though 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 called, leaving ZERO_OID set to empty, so when 'checkout
    main from invalid HEAD' runs 'echo $ZERO_OID >.git/HEAD' 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>
---
 t/test-lib.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)