rev-parse: rev-parse: add --is-shallow-repository

Running `git fetch --unshallow` on a repo that is not in fact shallow
produces a fatal error message. Add a helper to rev-parse that scripters
can use to determine whether a repo is shallow or not.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -235,6 +235,9 @@ print a message to stderr and exit with nonzero status.
 --is-bare-repository::
 	When the repository is bare print "true", otherwise "false".
 
+--is-shallow-repository::
+	When the repository is shallow print "true", otherwise "false".
+
 --resolve-git-dir <path>::
 	Check if <path> is a valid repository or a gitfile that
 	points at a valid repository, and print the location of the

builtin/rev-parse.c

@@ -868,6 +868,11 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 						: "false");
 				continue;
 			}
+			if (!strcmp(arg, "--is-shallow-repository")) {
+				printf("%s\n", is_repository_shallow() ? "true"
+						: "false");
+				continue;
+			}
 			if (!strcmp(arg, "--shared-index-path")) {
 				if (read_cache() < 0)
 					die(_("Could not read the index"));

t/t1500-rev-parse.sh

@@ -116,6 +116,21 @@ test_expect_success 'git-path inside sub-dir' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rev-parse --is-shallow-repository in shallow repo' '
+	test_commit test_commit &&
+	echo true >expect &&
+	git clone --depth 1 --no-local . shallow &&
+	test_when_finished "rm -rf shallow" &&
+	git -C shallow rev-parse --is-shallow-repository >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rev-parse --is-shallow-repository in non-shallow repo' '
+	echo false >expect &&
+	git rev-parse --is-shallow-repository >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'showing the superproject correctly' '
 	git rev-parse --show-superproject-working-tree >out &&
 	test_must_be_empty out &&