add: die if both --dry-run and --interactive are given

The interactive machinery does not obey --dry-run. Die appropriately
if both flags are passed.

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

builtin/add.c

diff --git a/builtin/add.c b/builtin/add.c
index afccf2f..5cea3dc 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -459,6 +459,8 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 	if (patch_interactive)
 		add_interactive = 1;
 	if (add_interactive) {
+		if (show_only)
+			die(_("--dry-run is incompatible with --interactive/--patch"));
 		if (pathspec_from_file)
 			die(_("--pathspec-from-file is incompatible with --interactive/--patch"));
 		exit(interactive_add(argv + 1, prefix, patch_interactive));

t/t3700-add.sh

diff --git a/t/t3700-add.sh b/t/t3700-add.sh
index b3b122f..171b323 100755
--- a/t/t3700-add.sh
+++ b/t/t3700-add.sh
@@ -343,6 +343,10 @@
 	test_cmp expect.err actual.err
 '
 
+test_expect_success 'git add --dry-run --interactive should fail' '
+	test_must_fail git add --dry-run --interactive
+'
+
 test_expect_success 'git add empty string should fail' '
 	test_must_fail git add ""
 '