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/t5534-push-signed.sh

@@ -273,4 +273,26 @@ test_expect_success GPGSM 'fail without key and heed user.signingkey x509' '
 	test_cmp expect dst/push-cert-status
 '
 
+test_expect_success GPG 'failed atomic push does not execute GPG' '
+	prepare_dst &&
+	git -C dst config receive.certnonceseed sekrit &&
+	write_script gpg <<-EOF &&
+	# should check atomic push locally before running GPG.
+	exit 1
+	EOF
+	test_must_fail env PATH="$TRASH_DIRECTORY:$PATH" git push \
+			--signed --atomic --porcelain \
+			dst noop ff noff >out 2>err &&
+
+	test_i18ngrep ! "gpg failed to sign" err &&
+	cat >expect <<-EOF &&
+	To dst
+	=	refs/heads/noop:refs/heads/noop	[up to date]
+	!	refs/heads/ff:refs/heads/ff	[rejected] (atomic push failed)
+	!	refs/heads/noff:refs/heads/noff	[rejected] (non-fast-forward)
+	Done
+	EOF
+	test_cmp expect out
+'
+
 test_done