builtin/hash-object: fix uninitialized hash function

builtin/hash-object: fix uninitialized hash function

The git-hash-object(1) command allows users to hash an object even
without a repository. Starting with c8aed5e8da (repository: stop setting
SHA1 as the default object hash, 2024-05-07), this will make us hit an
uninitialized hash function, which subsequently leads to a segfault.

Fix this by falling back to SHA-1 explicitly when running outside of
a Git repository. Users can use GIT_DEFAULT_HASH environment to
specify what hash algorithm they want, so arguably this code should
not be needed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>