config: complain about --worktree outside of a git repo

Running `git config --worktree` outside of a git repository hits a BUG()
when trying to enumerate the worktrees. Let's catch this error earlier
and die() with a friendlier message.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -628,11 +628,15 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		usage_builtin_config();
 	}
 
-	if (use_local_config && nongit)
-		die(_("--local can only be used inside a git repository"));
+	if (nongit) {
+		if (use_local_config)
+			die(_("--local can only be used inside a git repository"));
+		if (given_config_source.blob)
+			die(_("--blob can only be used inside a git repository"));
+		if (use_worktree_config)
+			die(_("--worktree can only be used inside a git repository"));
 
-	if (given_config_source.blob && nongit)
-		die(_("--blob can only be used inside a git repository"));
+	}
 
 	if (given_config_source.file &&
 			!strcmp(given_config_source.file, "-")) {

t/t1300-config.sh

@@ -1836,11 +1836,14 @@ test_expect_success '--show-scope with --show-origin' '
 	test_cmp expect output
 '
 
-test_expect_success '--local requires a repo' '
-	# we expect 128 to ensure that we do not simply
-	# fail to find anything and return code "1"
-	test_expect_code 128 nongit git config --local foo.bar
-'
+for opt in --local --worktree
+do
+	test_expect_success "$opt requires a repo" '
+		# we expect 128 to ensure that we do not simply
+		# fail to find anything and return code "1"
+		test_expect_code 128 nongit git config $opt foo.bar
+	'
+done
 
 cat >.git/config <<-\EOF &&
 [core]