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/t6425-merge-rename-delete.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+test_description='Merge-recursive rename/delete conflict message'
+. ./test-lib.sh
+
+test_expect_success 'rename/delete' '
+	echo foo >A &&
+	git add A &&
+	git commit -m "initial" &&
+
+	git checkout -b rename &&
+	git mv A B &&
+	git commit -m "rename" &&
+
+	git checkout master &&
+	git rm A &&
+	git commit -m "delete" &&
+
+	test_must_fail git merge --strategy=recursive rename >output &&
+	test_i18ngrep "CONFLICT (rename/delete): A.* renamed .*to B.* in rename" output &&
+	test_i18ngrep "CONFLICT (rename/delete): A.*deleted in HEAD." output
+'
+
+test_done