index-pack: complain when --stdin is used outside of a repo

The index-pack builtin is marked as RUN_SETUP_GENTLY,
because it's perfectly fine to index a pack in the
filesystem outside of any repository. However, --stdin mode
will write the result to the object database, which does not
make sense outside of a repository. Doing so creates a bogus
".git" directory with nothing in it except the newly-created
pack and its index.

Instead, let's flag this as an error and abort.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/index-pack.c

@@ -1730,6 +1730,8 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 		usage(index_pack_usage);
 	if (fix_thin_pack && !from_stdin)
 		die(_("--fix-thin cannot be used without --stdin"));
+	if (from_stdin && !startup_info->have_repository)
+		die(_("--stdin requires a git repository"));
 	if (!index_name && pack_name)
 		index_name = derive_filename(pack_name, ".idx", &index_name_buf);
 	if (keep_msg && !keep_name && pack_name)

t/t5300-pack-object.sh

@@ -406,6 +406,21 @@ test_expect_success 'verify resulting packs' '
 	git verify-pack test-11-*.pack
 '
 
+test_expect_success 'set up pack for non-repo tests' '
+	# make sure we have a pack with no matching index file
+	cp test-1-*.pack foo.pack
+'
+
+test_expect_success 'index-pack --stdin complains of non-repo' '
+	nongit test_must_fail git index-pack --stdin <foo.pack &&
+	test_path_is_missing non-repo/.git
+'
+
+test_expect_success 'index-pack <pack> works in non-repo' '
+	nongit git index-pack ../foo.pack &&
+	test_path_is_file foo.idx
+'
+
 #
 # WARNING!
 #