validate_headref: use get_oid_hex for detached HEADs

If a candidate HEAD isn't a symref, we check that it contains a viable sha1. But in a post-sha1 world, we should be checking whether it has any plausible object-id. We can do that by switching to get_oid_hex(). Note that both before and after this patch, we only check for a plausible object id at the start of the file, and then call that good enough. We ignore any content _after_ the hex, so a string like: 0123456789012345678901234567890123456789 foo is accepted. Though we do put extra bytes like this into some pseudorefs (e.g., FETCH_HEAD), we don't typically do so with HEAD. We could tighten this up by using parse_oid_hex(), like: if (!parse_oid_hex(buffer, &oid, &end) && *end++ == '\n' && *end == '\0') return 0; But we're probably better to remain on the loose side. We're just checking here for a plausible-looking repository directory, so heuristics are acceptable (if we really want to be meticulous, we should use the actual ref code to parse HEAD). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>