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.sh

@@ -455,6 +455,9 @@ export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
 export EDITOR
 
+GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-sha1}"
+export GIT_DEFAULT_HASH
+
 # Tests using GIT_TRACE typically don't want <timestamp> <file>:<line> output
 GIT_TRACE_BARE=1
 export GIT_TRACE_BARE
@@ -1689,7 +1692,11 @@ test_lazy_prereq CURL '
 # which will not work with other hash algorithms and tests that work but don't
 # test anything meaningful (e.g. special values which cause short collisions).
 test_lazy_prereq SHA1 '
-	test $(git hash-object /dev/null) = e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+	case "$GIT_DEFAULT_HASH" in
+	sha1) true ;;
+	"") test $(git hash-object /dev/null) = e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 ;;
+	*) false ;;
+	esac
 '
 
 test_lazy_prereq REBASE_P '