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