Check for sha256 support to use --object-format flag (#28928)

Check for sha256 support to use --object-format flag (#28928)

This should fix https://github.com/go-gitea/gitea/issues/28927

Technically older versions of Git would support this flag as well, but
per https://github.com/go-gitea/gitea/pull/28466 that's the version
where using it (object-format=sha256) left "experimental" state.

`sha1` is (currently) the default, so older clients should be unaffected
in either case.

Signed-off-by: jolheiser <john.olheiser@gmail.com>