Merge branch 'bc/use-sha256-by-default-in-3.0'

Merge branch 'bc/use-sha256-by-default-in-3.0'

Prepare to flip the default hash function to SHA-256.

* bc/use-sha256-by-default-in-3.0:
  Enable SHA-256 by default in breaking changes mode
  help: add a build option for default hash
  t5300: choose the built-in hash outside of a repo
  t4042: choose the built-in hash outside of a repo
  t1007: choose the built-in hash outside of a repo
  t: default to compile-time default hash if not set
  setup: use the default algorithm to initialize repo format
  Use legacy hash for legacy formats
  builtin: use default hash when outside a repository
  hash: add a constant for the legacy hash algorithm
  hash: add a constant for the default hash algorithm