Merge branch 'ps/undecided-is-not-necessarily-sha1' into jc/undecided-is-not-necessarily-sha1-fix

* ps/undecided-is-not-necessarily-sha1: repository: stop setting SHA1 as the default object hash oss-fuzz/commit-graph: set up hash algorithm builtin/shortlog: don't set up revisions without repo builtin/diff: explicitly set hash algo when there is no repo builtin/bundle: abort "verify" early when there is no repository builtin/blame: don't access potentially unitialized `the_hash_algo` builtin/rev-parse: allow shortening to more than 40 hex characters remote-curl: fix parsing of detached SHA256 heads attr: fix BUG() when parsing attrs outside of repo attr: don't recompute default attribute source parse-options-cb: only abbreviate hashes when hash algo is known path: move `validate_headref()` to its only user path: harden validation of HEAD with non-standard hashes