show-index: warn when falling back to SHA-1 outside a repository

show-index: warn when falling back to SHA-1 outside a repository

When 'git show-index' is run outside of a repository and no hashing
algorithm is specified via --object-format, it silently falls back
to SHA-1, relying on the historical default.

This works for existing SHA-1 based index files, but the behavior can
be ambiguous and confusing when the input index file uses a different
hash algorithm, such as SHA-256.

Add a warning when this fallback happens to make the assumption
explicit and to guide users toward using --object-format when needed.

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>