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/t4134-apply-submodule.sh

@@ -15,7 +15,7 @@ index 0000000..0123456
 --- /dev/null
 +++ b/dir/sm
 @@ -0,0 +1 @@
-+Subproject commit 0123456789abcdef0123456789abcdef01234567
++Subproject commit $(test_oid numeric)
 EOF
 	cat > remove-sm.patch <<EOF
 diff --git a/dir/sm b/dir/sm
@@ -24,7 +24,7 @@ index 0123456..0000000
 --- a/dir/sm
 +++ /dev/null
 @@ -1 +0,0 @@
--Subproject commit 0123456789abcdef0123456789abcdef01234567
+-Subproject commit $(test_oid numeric)
 EOF
 '