Merge branch 'ps/undecided-is-not-necessarily-sha1'

Merge branch 'ps/undecided-is-not-necessarily-sha1'

Before discovering the repository details, We used to assume SHA-1
as the "default" hash function, which has been corrected. Hopefully
this will smoke out codepaths that rely on such an unwarranted
assumptions.

* 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