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/t3506-cherry-pick-ff.sh

@@ -16,7 +16,11 @@ test_expect_success setup '
 	git add file1 &&
 	test_tick &&
 	git commit -m "second" &&
-	git tag second
+	git tag second &&
+	test_oid_cache <<-EOF
+	cp_ff sha1:1df192cd8bc58a2b275d842cede4d221ad9000d1
+	cp_ff sha256:e70d6b7fc064bddb516b8d512c9057094b96ce6ff08e12080acc4fe7f1d60a1d
+	EOF
 '
 
 test_expect_success 'cherry-pick using --ff fast forwards' '
@@ -102,7 +106,7 @@ test_expect_success 'cherry pick a root commit with --ff' '
 	git add file2 &&
 	git commit --amend -m "file2" &&
 	git cherry-pick --ff first &&
-	test "$(git rev-parse --verify HEAD)" = "1df192cd8bc58a2b275d842cede4d221ad9000d1"
+	test "$(git rev-parse --verify HEAD)" = "$(test_oid cp_ff)"
 '
 
 test_expect_success 'cherry-pick --ff on unborn branch' '