fast-import: add 'sign-if-invalid' mode to '--signed-tags=<mode>'
With
ee66c793f8 (fast-import: add mode to sign commits with invalid
signatures, 2026-03-12), git-fast-import(1) learned to verify commit
signatures during import and replace signatures that fail verification
with a newly generated one. Extend the same behavior to signed tag
objects by introducing a 'sign-if-invalid' mode for the '--signed-tags'
option.
Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>