t1092: stress test 'git sparse-checkout set'

The 'sparse-index contents' test checks that the sparse index has the
correct set of sparse directories in the index after modifying the cone
mode patterns using 'git sparse-checkout set'. Add to the coverage here
by adding more complicated scenarios that were not previously tested.

In order to check paths that do not exist at HEAD, we need to modify the
test_sparse_checkout_set helper slightly:

1. Add the --skip-checks argument to the 'set' command to avoid failures
   when passing paths that do not exist at HEAD.

2. When looking for the non-existence of sparse directories for the
   paths in $CONE_DIRS, allow the rev-list command to fail because the
   path does not exist at HEAD.

This allows us to add some interesting test cases.

Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1092-sparse-checkout-compatibility.sh

@@ -212,7 +212,7 @@ test_sparse_unstaged () {
 test_sparse_checkout_set () {
 	CONE_DIRS=$1 &&
 	SPARSE_DIRS=$2 &&
-	git -C sparse-index sparse-checkout set $CONE_DIRS &&
+	git -C sparse-index sparse-checkout set --skip-checks $CONE_DIRS &&
 	git -C sparse-index ls-files --sparse --stage >cache &&
 
 	# Check that the directories outside of the sparse-checkout cone
@@ -228,7 +228,9 @@ test_sparse_checkout_set () {
 	# are not sparse directory entries.
 	for dir in $CONE_DIRS
 	do
-		TREE=$(git -C sparse-index rev-parse HEAD:$dir) &&
+		# Allow TREE to not exist because
+		# $dir does not exist at HEAD.
+		TREE=$(git -C sparse-index rev-parse HEAD:$dir) ||
 		! grep "040000 $TREE 0	$dir/" cache \
 			|| return 1
 	done
@@ -253,6 +255,19 @@ test_expect_success 'sparse-index contents' '
 		"deep/deeper2 folder1 folder2 x" \
 		"before deep/deeper1" &&
 
+	# Replace deep/deeper2 with deep/deeper1
+	# Replace folder1 with folder1/0/0
+	# Replace folder2 with non-existent folder2/2/3
+	# Add non-existent "bogus"
+	test_sparse_checkout_set \
+		"bogus deep/deeper1 folder1/0/0 folder2/2/3 x" \
+		"before deep/deeper2 folder2/0" &&
+
+	# Drop down to only files at root
+	test_sparse_checkout_set \
+		"" \
+		"before deep folder1 folder2 x" &&
+
 	# Disabling the sparse-index replaces tree entries with full ones
 	git -C sparse-index sparse-checkout init --no-sparse-index &&
 	test_sparse_match git ls-files --stage --sparse