remote-curl: fix parsing of detached SHA256 heads

The dumb HTTP transport tries to read the remote HEAD reference by downloading the "HEAD" file and then parsing it via `http_fetch_ref()`. This function will either parse the file as an object ID in case it is exactly `the_hash_algo->hexsz` long, or otherwise it will check whether the reference starts with "ref :" and parse it as a symbolic ref. This is broken when parsing detached HEADs of a remote SHA256 repository because we never update `the_hash_algo` to the discovered remote object hash. Consequently, `the_hash_algo` will always be the fallback SHA1 hash algorithm, which will cause us to fail parsing HEAD altogteher when it contains a SHA256 object ID. Fix this issue by setting up `the_hash_algo` via `repo_set_hash_algo()`. While at it, let's make the expected SHA1 fallback explicit in our code, which also addresses an upcoming issue where we are going to remove the SHA1 fallback for `the_hash_algo`. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>