remote-curl: fall back to default hash outside repo

When a remote helper like git-remote-http is invoked outside of a repository (for example, by running git ls-remote in a non-git directory), setup_git_directory_gently() leaves the_hash_algo uninitialized as NULL. If the user has a globally configured fetch refspec, remote-curl attempts to parse it during initialization. Inside parse_refspec(), it checks whether the LHS of the refspec is an exact OID by evaluating llen == the_hash_algo->hexsz. Because the_hash_algo is NULL, this results in a segmentation fault. In 9e89dcb66a (builtin/ls-remote: fall back to SHA1 outside of a repo, 2024-08-02), we added a workaround to ls-remote to fall back to the default hash algorithm to prevent exactly this type of crash when parsing refspec capabilities. However, because remote-curl runs as a separate process, it does not inherit that fallback and crashes anyway. Instead of pushing a NULL-guard workaround down into parse_refspec(), fix this by mirroring the ls-remote workaround directly in remote-curl.c. If we are operating outside a repository, initialize the_hash_algo to GIT_HASH_DEFAULT. This keeps the HTTP transport consistent with non-HTTP transports that execute in-process, preventing crashes without altering the generic refspec parsing logic. Reported-by: Jo Liss <joliss@gmail.com> Helped-by: Jeff King <peff@peff.net> Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>