test-lib: allow bare repository access when breaking changes are enabled

test-lib: allow bare repository access when breaking changes are enabled

A future patch will change the `safe.bareRepository` default from
`all` to `explicit` under `WITH_BREAKING_CHANGES`. At that point,
every test that operates on a bare repository through implicit
discovery would fail, regardless of whether the test is actually
about discovery or about how a specific command behaves once inside
a bare repository.

The maintainer suggested [1] setting `safe.bareRepository=all` in
the test environment's global config whenever `WITH_BREAKING_CHANGES`
is in effect, rather than adjusting each affected test to access
bare repositories explicitly (via `--git-dir`, `GIT_DIR`, or
similar). This means the test suite continues to exercise only the
historical default behavior even after the user-facing default
changes, relying on a small number of dedicated tests in t0035 to
validate the new, stricter default.

Since `$HOME` points at the trash directory (which doubles as the
test repository's working tree), writing to `$HOME/.gitconfig` also
creates a file inside the working tree. Exclude it via
`.git/info/exclude` to limit the fallout, though this does not
help tests that use `git ls-files --others` without
`--exclude-standard` or `git status --ignored`; those are addressed
by subsequent commits.

The `.git/info/exclude` write is guarded by `test -d .git/info`
rather than using `mkdir -p`, because some tests (e.g. t0008)
expect to create `.git/info/` themselves and would fail with
Patrick Steinhardt's `set -e` preparation (ps/test-set-e-clean) if
the directory already existed. For tests using `TEST_NO_CREATE_REPO`
(where no `.git/` exists at all), the guard also handles that case.

[1] https://lore.kernel.org/git/xmqqse98cc51.fsf@gitster.g/

Original-patch-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>