status tests: filter `.gitconfig` from status output

status tests: filter `.gitconfig` from status output

Since test-lib.sh creates `$HOME/.gitconfig` when
`WITH_BREAKING_CHANGES` is in effect, the file appears in `git
status` output as either untracked (`?? .gitconfig`) or ignored
(`!! .gitconfig` / `! .gitconfig`, depending on porcelain version),
because the `.git/info/exclude` entry causes git to treat it as an
ignored file rather than hiding it entirely.

In t7061 and t7521, which are pervasively affected, introduce a
`filter_gitconfig` helper that strips all status-prefix variants of
`.gitconfig` from the output before comparison. In the remaining
scripts (t7060, t7064, t7508), apply targeted adjustments.

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