t5601: restore `.gitconfig` after includeIf test

t5601: restore `.gitconfig` after includeIf test

One test in t5601 overwrites `$HOME/.gitconfig` with an `includeIf`
configuration snippet and removes the file in its cleanup. This
destroys the `safe.bareRepository=all` entry that test-lib.sh
writes when `WITH_BREAKING_CHANGES` is in effect, causing later
tests that use `git -C <bare-repo> config` to fail with "not in a
git directory".

Back up `.gitconfig` before overwriting and restore it in the
cleanup, so the global config survives into subsequent tests.

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