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

["SZEDER Gábor" <szeder.dev@gmail.com>]

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(-)

diff --git a/t/test-lib.sh b/t/test-lib.sh
index abcfbed6d6..a1e0182c2f 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1343,7 +1343,8 @@ fi
 GIT_TEMPLATE_DIR="$GIT_BUILD_DIR"/templates/blt
 GIT_CONFIG_NOSYSTEM=1
 GIT_ATTR_NOSYSTEM=1
-export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM
+GIT_CEILING_DIRECTORIES="$GIT_BUILD_DIR"
+export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM GIT_CEILING_DIRECTORIES
 
 if test -z "$GIT_TEST_CMP"
 then
-- 
2.33.0.358.g803110d36e