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/t6413-merge-crlf.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='merge conflict in crlf repo
+
+		b---M
+	       /   /
+	initial---a
+
+'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	git config core.autocrlf true &&
+	echo foo | append_cr >file &&
+	git add file &&
+	git commit -m "Initial" &&
+	git tag initial &&
+	git branch side &&
+	echo line from a | append_cr >file &&
+	git commit -m "add line from a" file &&
+	git tag a &&
+	git checkout side &&
+	echo line from b | append_cr >file &&
+	git commit -m "add line from b" file &&
+	git tag b &&
+	git checkout master
+'
+
+test_expect_success 'Check "ours" is CRLF' '
+	git reset --hard initial &&
+	git merge side -s ours &&
+	cat file | remove_cr | append_cr >file.temp &&
+	test_cmp file file.temp
+'
+
+test_expect_success 'Check that conflict file is CRLF' '
+	git reset --hard a &&
+	test_must_fail git merge side &&
+	cat file | remove_cr | append_cr >file.temp &&
+	test_cmp file file.temp
+'
+
+test_done