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/t3900-i18n-commit.sh

@@ -204,4 +204,41 @@ test_commit_autosquash_flags eucJP fixup
 
 test_commit_autosquash_flags ISO-2022-JP squash
 
+test_commit_autosquash_multi_encoding () {
+	flag=$1
+	old=$2
+	new=$3
+	msg=$4
+	test_expect_success "commit --$flag into $old from $new" '
+		git checkout -b $flag-$old-$new C0 &&
+		git config i18n.commitencoding $old &&
+		echo $old >>F &&
+		git commit -a -F "$TEST_DIRECTORY"/t3900/$msg &&
+		test_tick &&
+		echo intermediate stuff >>G &&
+		git add G &&
+		git commit -a -m "intermediate commit" &&
+		test_tick &&
+		git config i18n.commitencoding $new &&
+		echo $new-$flag >>F &&
+		git commit -a --$flag HEAD^ &&
+		git rebase --autosquash -i HEAD^^^ &&
+		git rev-list HEAD >actual &&
+		test_line_count = 3 actual &&
+		iconv -f $old -t UTF-8 "$TEST_DIRECTORY"/t3900/$msg >expect &&
+		if test $flag = squash; then
+			subject="$(head -1 expect)" &&
+			printf "\nsquash! %s\n" "$subject" >>expect
+		fi &&
+		git cat-file commit HEAD^ >raw &&
+		(sed "1,/^$/d" raw | iconv -f $new -t utf-8) >actual &&
+		test_cmp expect actual
+	'
+}
+
+test_commit_autosquash_multi_encoding fixup UTF-8 ISO-8859-1 1-UTF-8.txt
+test_commit_autosquash_multi_encoding squash ISO-8859-1 UTF-8 ISO8859-1.txt
+test_commit_autosquash_multi_encoding squash eucJP ISO-2022-JP eucJP.txt
+test_commit_autosquash_multi_encoding fixup ISO-2022-JP UTF-8 ISO-2022-JP.txt
+
 test_done