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/ci/run-docker.sh

@@ -0,0 +1,47 @@
+#!/bin/sh
+#
+# Download and run Docker image to build and test Git
+#
+
+. ${0%/*}/lib.sh
+
+case "$jobname" in
+Linux32)
+	CI_CONTAINER="daald/ubuntu32:xenial"
+	;;
+linux-musl)
+	CI_CONTAINER=alpine
+	;;
+*)
+	exit 1
+	;;
+esac
+
+docker pull "$CI_CONTAINER"
+
+# Use the following command to debug the docker build locally:
+# <host-user-id> must be 0 if podman is used as drop-in replacement for docker
+# $ docker run -itv "${PWD}:/usr/src/git" --entrypoint /bin/sh "$CI_CONTAINER"
+# root@container:/# export jobname=<jobname>
+# root@container:/# /usr/src/git/ci/run-docker-build.sh <host-user-id>
+
+container_cache_dir=/tmp/travis-cache
+
+docker run \
+	--interactive \
+	--env DEVELOPER \
+	--env DEFAULT_TEST_TARGET \
+	--env GIT_PROVE_OPTS \
+	--env GIT_TEST_OPTS \
+	--env GIT_TEST_CLONE_2GB \
+	--env MAKEFLAGS \
+	--env jobname \
+	--env cache_dir="$container_cache_dir" \
+	--volume "${PWD}:/usr/src/git" \
+	--volume "$cache_dir:$container_cache_dir" \
+	"$CI_CONTAINER" \
+	/usr/src/git/ci/run-docker-build.sh $(id -u $USER)
+
+check_unignored_build_artifacts
+
+save_good_tree