t1305: use `--git-dir=.` for bare repo in include cycle test

t1305: use `--git-dir=.` for bare repo in include cycle test

Earlier tests in t1305 overwrite `$HOME/.gitconfig` with their own
content as part of testing config includes. This clobbers the
`safe.bareRepository=all` entry that test-lib.sh writes when
`WITH_BREAKING_CHANGES` is in effect, causing `git -C cycle config`
to fail with "not in a git directory" when it tries to access the
bare repository created by `git init --bare cycle`.

Use `--git-dir=.` to access the bare repo explicitly, avoiding the
dependency on global config for repository discovery.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>