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/t3418-rebase-continue.sh

@@ -120,6 +120,20 @@ test_expect_success REBASE_P 'rebase passes merge strategy options correctly' '
 	git rebase --continue
 '
 
+test_expect_success 'rebase -r passes merge strategy options correctly' '
+	rm -fr .git/rebase-* &&
+	git reset --hard commit-new-file-F3-on-topic-branch &&
+	test_commit merge-theirs &&
+	git reset --hard HEAD^ &&
+	test_commit some-other-commit &&
+	test_tick &&
+	git merge --no-ff merge-theirs &&
+	FAKE_LINES="1 3 edit 4 5 7 8 9" git rebase -i -f -r -m \
+		-s recursive --strategy-option=theirs HEAD~2 &&
+	test_commit force-change-ours &&
+	git rebase --continue
+'
+
 test_expect_success '--skip after failed fixup cleans commit message' '
 	test_when_finished "test_might_fail git rebase --abort" &&
 	git checkout -b with-conflicting-fixup &&