rev-parse: allow printing compatibility hash

Right now, we have a way to print the storage hash, the input hash, and
the output hash, but we lack a way to print the compatibility hash.  Add
a new type to --show-object-format, compat, which prints this value.

If no compatibility hash exists, simply print a newline.  This is
important to allow users to use multiple options at once while still
getting unambiguous output.

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

Documentation/git-rev-parse.adoc

@@ -324,11 +324,12 @@ The following options are unaffected by `--path-format`:
 	path of the current directory relative to the top-level
 	directory.
 
---show-object-format[=(storage|input|output)]::
+--show-object-format[=(storage|input|output|compat)]::
-	Show the object format (hash algorithm) used for the repository
+	Show the object format (hash algorithm) used for the repository for storage
-	for storage inside the `.git` directory, input, or output. For
+	inside the `.git` directory, input, output, or compatibility. For input,
-	input, multiple algorithms may be printed, space-separated.
+	multiple algorithms may be printed, space-separated. If `compat` is
-	If not specified, the default is "storage".
+	requested and no compatibility algorithm is enabled, prints an empty line. If
+	not specified, the default is "storage".
 
 --show-ref-format::
 	Show the reference storage format used for the repository.

builtin/rev-parse.c

@@ -1108,11 +1108,20 @@ int cmd_rev_parse(int argc,
 				const char *val = arg ? arg : "storage";
 
 				if (strcmp(val, "storage") &&
+				    strcmp(val, "compat") &&
 				    strcmp(val, "input") &&
 				    strcmp(val, "output"))
 					die(_("unknown mode for --show-object-format: %s"),
 					    arg);
-				puts(the_hash_algo->name);
+
+				if (!strcmp(val, "compat")) {
+					if (the_repository->compat_hash_algo)
+						puts(the_repository->compat_hash_algo->name);
+					else
+						putchar('\n');
+				} else {
+					puts(the_hash_algo->name);
+				}
 				continue;
 			}
 			if (!strcmp(arg, "--show-ref-format")) {

t/t1500-rev-parse.sh

@@ -207,6 +207,40 @@ test_expect_success 'rev-parse --show-object-format in repo' '
 	grep "unknown mode for --show-object-format: squeamish-ossifrage" err
 '
 
+
+test_expect_success 'rev-parse --show-object-format in repo with compat mode' '
+	mkdir repo &&
+	(
+		sane_unset GIT_DEFAULT_HASH &&
+		cd repo &&
+		git init --object-format=sha256 &&
+		git config extensions.compatobjectformat sha1 &&
+		echo sha256 >expect &&
+		git rev-parse --show-object-format >actual &&
+		test_cmp expect actual &&
+		git rev-parse --show-object-format=storage >actual &&
+		test_cmp expect actual &&
+		git rev-parse --show-object-format=input >actual &&
+		test_cmp expect actual &&
+		git rev-parse --show-object-format=output >actual &&
+		test_cmp expect actual &&
+		echo sha1 >expect &&
+		git rev-parse --show-object-format=compat >actual &&
+		test_cmp expect actual &&
+		test_must_fail git rev-parse --show-object-format=squeamish-ossifrage 2>err &&
+		grep "unknown mode for --show-object-format: squeamish-ossifrage" err
+	) &&
+	mkdir repo2 &&
+	(
+		sane_unset GIT_DEFAULT_HASH &&
+		cd repo2 &&
+		git init --object-format=sha256 &&
+		echo >expect &&
+		git rev-parse --show-object-format=compat >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'rev-parse --show-ref-format' '
 	test_detect_ref_format >expect &&
 	git rev-parse --show-ref-format >actual &&