merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/t/t7420-submodule-set-url.sh

@@ -0,0 +1,55 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Denton Liu
+#
+
+test_description='Test submodules set-url subcommand
+
+This test verifies that the set-url subcommand of git-submodule is working
+as expected.
+'
+
+TEST_NO_CREATE_REPO=1
+. ./test-lib.sh
+
+test_expect_success 'submodule config cache setup' '
+	mkdir submodule &&
+	(
+		cd submodule &&
+		git init &&
+		echo a >file &&
+		git add file &&
+		git commit -ma
+	) &&
+	mkdir super &&
+	(
+		cd super &&
+		git init &&
+		git submodule add ../submodule &&
+		git commit -m "add submodule"
+	)
+'
+
+test_expect_success 'test submodule set-url' '
+	# add a commit and move the submodule (change the url)
+	(
+		cd submodule &&
+		echo b >>file &&
+		git add file &&
+		git commit -mb
+	) &&
+	mv submodule newsubmodule &&
+
+	git -C newsubmodule show >expect &&
+	(
+		cd super &&
+		test_must_fail git submodule update --remote &&
+		git submodule set-url submodule ../newsubmodule &&
+		grep -F "url = ../newsubmodule" .gitmodules &&
+		git submodule update --remote
+	) &&
+	git -C super/submodule show >actual &&
+	test_cmp expect actual
+'
+
+test_done