merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/t/t4137-apply-submodule.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+test_description='git apply handling submodules'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-submodule-update.sh
+
+apply_index () {
+	git diff --ignore-submodules=dirty "..$1" | git apply --index -
+}
+
+test_submodule_switch "apply_index"
+
+apply_3way () {
+	git diff --ignore-submodules=dirty "..$1" | git apply --3way -
+}
+
+test_submodule_switch "apply_3way"
+
+test_done