shallow: fix relative deepen on non-shallow repositories

shallow: fix relative deepen on non-shallow repositories

The commit "3ef68ff40e (shallow: handling fetch relative-deepen,
2026-02-15)" introduced a bug where using --deepen=<n> on a non-
shallow repository incorrectly treated the value as an absolute
depth, resulting in a shallow fetch and truncated history.

This patch prevents any modification when a relative deepen is
requested on a non-shallow repository.

A test is added to ensure that history is not changed when
--deepen is used on a non-shallow repository.

Reported-by: Owen Stephens <owen@owenstephens.co.uk>
Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>