Allow specifying compatibility hash

We want to specify a compatibility hash for testing interactions for
SHA-256 repositories where we have SHA-1 compatibility enabled.  Allow
the user to specify this scenario in the test suite by setting
GIT_TEST_DEFAULT_HASH to "sha256:sha1".

Note that this will get passed into GIT_DEFAULT_HASH, which Git itself
does not presently support.  However, we will support this in a future
commit.

Since we'll now want to know the value for a specific version, let's add
the ability to specify either the storage hash (in this case, SHA-256)
or the compatibility hash (SHA-1).  We use a different value for the
compatibility hash that will be enabled for all repositories
(test_repo_compat_hash_algo) versus the one that is used individually in
some tests (test_compat_hash_algo), since we want to still run those
individual tests without requiring that the testsuite be run fully in a
compatibility mode.

Finally, in this scenario, we can no longer rely on having broken
objects work since we lack compatibility mappings to rewrite objects in
the repository.  Add a prerequisite, BROKEN_OBJECTS, that checks to see
if creating deliberately broken objects is possible, so that we can
disable these tests if not.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bk2204

t/test-lib-functions.sh

@@ -1708,11 +1708,16 @@ test_set_hash () {
 # Detect the hash algorithm in use.
 test_detect_hash () {
 	case "${GIT_TEST_DEFAULT_HASH:-$GIT_TEST_BUILTIN_HASH}" in
-	"sha256")
+	*:*)
+	    test_hash_algo="${GIT_TEST_DEFAULT_HASH%%:*}"
+	    test_compat_hash_algo="${GIT_TEST_DEFAULT_HASH##*:}"
+	    test_repo_compat_hash_algo="$test_compat_hash_algo"
+	    ;;
+	sha256)
 	    test_hash_algo=sha256
 	    test_compat_hash_algo=sha1
 	    ;;
-	*)
+	sha1)
 	    test_hash_algo=sha1
 	    test_compat_hash_algo=sha256
 	    ;;

t/test-lib.sh

@@ -1917,6 +1917,13 @@ test_lazy_prereq DEFAULT_HASH_ALGORITHM '
 test_lazy_prereq DEFAULT_REPO_FORMAT '
 	test_have_prereq SHA1,REFFILES
 '
+# BROKEN_OBJECTS is a test if we can write deliberately broken objects and
+# expect them to work.  When running using SHA-256 mode with SHA-1
+# compatibility, we cannot write such objects because there's no SHA-1
+# compatibility value for a nonexistent object.
+test_lazy_prereq BROKEN_OBJECTS '
+	test -z "$test_repo_compat_hash_algo"
+'
 
 # Ensure that no test accidentally triggers a Git command
 # that runs the actual maintenance scheduler, affecting a user's