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

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

third_party/git/t/t2105-update-index-gitfile.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Brad King
+#
+
+test_description='git update-index for gitlink to .git file.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'submodule with absolute .git file' '
+	mkdir sub1 &&
+	(cd sub1 &&
+	 git init &&
+	 REAL="$(pwd)/.real" &&
+	 mv .git "$REAL" &&
+	 echo "gitdir: $REAL" >.git &&
+	 test_commit first)
+'
+
+test_expect_success 'add gitlink to absolute .git file' '
+	git update-index --add -- sub1
+'
+
+test_expect_success 'submodule with relative .git file' '
+	mkdir sub2 &&
+	(cd sub2 &&
+	 git init &&
+	 mv .git .real &&
+	 echo "gitdir: .real" >.git &&
+	 test_commit first)
+'
+
+test_expect_success 'add gitlink to relative .git file' '
+	git update-index --add -- sub2
+'
+
+test_done