Merge branch 'bc/sha-256-part-3'

The final leg of SHA-256 transition.

* bc/sha-256-part-3: (39 commits)
  t: remove test_oid_init in tests
  docs: add documentation for extensions.objectFormat
  ci: run tests with SHA-256
  t: make SHA1 prerequisite depend on default hash
  t: allow testing different hash algorithms via environment
  t: add test_oid option to select hash algorithm
  repository: enable SHA-256 support by default
  setup: add support for reading extensions.objectformat
  bundle: add new version for use with SHA-256
  builtin/verify-pack: implement an --object-format option
  http-fetch: set up git directory before parsing pack hashes
  t0410: mark test with SHA1 prerequisite
  t5308: make test work with SHA-256
  t9700: make hash size independent
  t9500: ensure that algorithm info is preserved in config
  t9350: make hash size independent
  t9301: make hash size independent
  t9300: use $ZERO_OID instead of hard-coded object ID
  t9300: abstract away SHA-1-specific constants
  t8011: make hash size independent
  ...

t/test-lib-functions.sh

@@ -1464,9 +1464,7 @@ test_set_hash () {
 
 # Detect the hash algorithm in use.
 test_detect_hash () {
-	# Currently we only support SHA-1, but in the future this function will
-	# actually detect the algorithm in use.
-	test_hash_algo='sha1'
+	test_hash_algo="${GIT_TEST_DEFAULT_HASH:-sha1}"
 }
 
 # Load common hash metadata and common placeholder object IDs for use with
@@ -1515,7 +1513,17 @@ test_oid_cache () {
 # Look up a per-hash value based on a key ($1).  The value must have been loaded
 # by test_oid_init or test_oid_cache.
 test_oid () {
-	local var="test_oid_${test_hash_algo}_$1" &&
+	local algo="${test_hash_algo}" &&
+
+	case "$1" in
+	--hash=*)
+		algo="${1#--hash=}" &&
+		shift;;
+	*)
+		;;
+	esac &&
+
+	local var="test_oid_${algo}_$1" &&
 
 	# If the variable is unset, we must be missing an entry for this
 	# key-hash pair, so exit with an error.