Re: [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 $GIT_BUILD_DIR.
>  - Minor commit message tweaks.
>
>  t/test-lib.sh | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/t/test-lib.sh b/t/test-lib.sh
> index abcfbed6d6..fc1e521519 100644
> --- a/t/test-lib.sh
> +++ b/t/test-lib.sh
> @@ -1343,7 +1343,8 @@ fi
>  GIT_TEMPLATE_DIR="$GIT_BUILD_DIR"/templates/blt
>  GIT_CONFIG_NOSYSTEM=1
>  GIT_ATTR_NOSYSTEM=1
> -export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM
> +GIT_CEILING_DIRECTORIES="$TRASH_DIRECTORY/.."
> +export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM GIT_CEILING_DIRECTORIES
>  
>  if test -z "$GIT_TEST_CMP"
>  then