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/t7012-skip-worktree-writing.sh

@@ -134,6 +134,21 @@ test_expect_success 'git-clean, dirty case' '
 	test_i18ncmp expected result
 '
 
+test_expect_success '--ignore-skip-worktree-entries leaves worktree alone' '
+	test_commit keep-me &&
+	git update-index --skip-worktree keep-me.t &&
+	rm keep-me.t &&
+
+	: ignoring the worktree &&
+	git update-index --remove --ignore-skip-worktree-entries keep-me.t &&
+	git diff-index --cached --exit-code HEAD &&
+
+	: not ignoring the worktree, a deletion is staged &&
+	git update-index --remove keep-me.t &&
+	test_must_fail git diff-index --cached --exit-code HEAD \
+		--diff-filter=D -- keep-me.t
+'
+
 #TODO test_expect_failure 'git-apply adds file' false
 #TODO test_expect_failure 'git-apply updates file' false
 #TODO test_expect_failure 'git-apply removes file' false