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/t5509-fetch-push-namespaces.sh

@@ -20,7 +20,7 @@ test_expect_success setup '
 	) &&
 	commit0=$(cd original && git rev-parse HEAD^) &&
 	commit1=$(cd original && git rev-parse HEAD) &&
-	git init pushee &&
+	git init --bare pushee &&
 	git init puller
 '
 
@@ -152,4 +152,15 @@ test_expect_success 'clone chooses correct HEAD (v2)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'denyCurrentBranch and unborn branch with ref namespace' '
+	(
+		cd original &&
+		git init unborn &&
+		git remote add unborn-namespaced "ext::git --namespace=namespace %s unborn" &&
+		test_must_fail git push unborn-namespaced HEAD:master &&
+		git -C unborn config receive.denyCurrentBranch updateInstead &&
+		git push unborn-namespaced HEAD:master
+	)
+'
+
 test_done