t: default to compile-time default hash if not set

Right now, the default compile-time hash is SHA-1.  However, in the
future, this might change and it would be helpful to gracefully handle
this case in our testsuite.

To avoid making these assumptions, let's introduce a variable that
contains the built-in default hash and use it in our setup code as the
fallback value if no hash was explicitly set.  For now, this is always
SHA-1, but in a future commit, we'll allow adjusting this and the
variable will be more useful.

To allow us to make our tests more robust, allow test_oid to take the
--hash=builtin option to specify this hash, whatever it is.

Additionally, add a DEFAULT_HASH_ALGORITHM prerequisite to check for the
compile-time hash.

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

t/test-lib-functions.sh

@@ -1695,7 +1695,7 @@ test_set_hash () {
 
 # Detect the hash algorithm in use.
 test_detect_hash () {
-	case "$GIT_TEST_DEFAULT_HASH" in
+	case "${GIT_TEST_DEFAULT_HASH:-$GIT_TEST_BUILTIN_HASH}" in
 	"sha256")
 	    test_hash_algo=sha256
 	    test_compat_hash_algo=sha1
@@ -1767,6 +1767,9 @@ test_oid () {
 	--hash=compat)
 		algo="$test_compat_hash_algo" &&
 		shift;;
+	--hash=builtin)
+		algo="$GIT_TEST_BUILTIN_HASH" &&
+		shift;;
 	--hash=*)
 		algo="${1#--hash=}" &&
 		shift;;