t2041: test symlink nested through another symlink

Add a regression test for the grafting fix: a symlink whose target
passes through another symlink should still become a directory
symlink once its real target appears, even when that target is only
populated after the leading symlink has already resolved.

Windows path resolution follows a symlink's target regardless of
whether it is flagged as a file or directory symlink, so opening a
path through it succeeds either way; the type only becomes visible in
things like `cmd.exe /c dir`, which marks directory symlinks as
<SYMLINKD> and file symlinks as <SYMLINK>. The test asserts on that
distinction rather than on read access, since the latter would pass
even without the previous commit's fix.

Signed-off-by: Chris Harris <chris.harris@discordapp.com>

Author: chrisharris-discord

t/t2041-checkout-symlink-through-symlink.sh

@@ -0,0 +1,56 @@
+#!/bin/sh
+
+test_description='checkout a symlink nested through another symlink on Windows
+
+A phantom symlink may target a path that goes through another
+symlink. Ensures that such a symlink is still upgraded to a directory
+symlink once its real target appears, even when that target directory
+is only populated after the leading symlink has already resolved.'
+
+# Tell MSYS to create native symlinks. Without this flag test-lib's
+# prerequisite detection for SYMLINKS doesn't detect the right thing.
+MSYS=winsymlinks:nativestrict && export MSYS
+
+. ./test-lib.sh
+
+if ! test_have_prereq MINGW,SYMLINKS
+then
+	skip_all='skipping $0: MinGW-only test, which requires symlink support.'
+	test_done
+fi
+
+# Adds a symlink to the index without clobbering the work tree.
+cache_symlink () {
+	sha=$(printf '%s' "$1" | git hash-object --stdin -w) &&
+	git update-index --add --cacheinfo 120000,$sha,"$2"
+}
+
+# Adds a regular file to the index without clobbering the work tree.
+cache_file () {
+	sha=$(printf '%s' "$1" | git hash-object --stdin -w) &&
+	git update-index --add --cacheinfo 100644,$sha,"$2"
+}
+
+test_expect_success 'symlink nested through another symlink resolves' '
+	test_when_finished "rm -rf chained" &&
+	mkdir chained &&
+	(
+		cd chained &&
+		git init -q &&
+
+		cache_symlink realdir leading &&
+		cache_symlink leading/sub nested &&
+		cache_file content realdir/sub/file &&
+
+		git checkout -- . &&
+
+		# "cmd.exe /c dir" marks directory symlinks as <SYMLINKD>
+		# and file symlinks as <SYMLINK>; unlike opening a path
+		# through the symlink, this distinguishes the two even
+		# though both resolve identically for plain reads.
+		cmd.exe //c dir . >dir-listing &&
+		grep "SYMLINKD.*nested" dir-listing
+	)
+'
+
+test_done