diff(sparse-index): verify with partially-sparse

This verifies that `diff` and `diff --staged` behave the same in sparse
index repositories in the following partially-staged scenarios (i.e. the
index, HEAD, and working directory differ at a given path):
    1. Path is within sparse-checkout cone.
    2. Path is outside sparse-checkout cone.
    3. A merge conflict exists for paths outside sparse-checkout cone.

Signed-off-by: Lessley Dennington <lessleydennington@gmail.com>

t/t1092-sparse-checkout-compatibility.sh

@@ -505,6 +505,43 @@ test_expect_success 'diff --cached' '
 	test_all_match git diff --cached
 '
 
+test_expect_success 'diff partially-staged' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>$1
+	EOF
+
+	# Add file within cone
+	test_all_match git sparse-checkout set deep &&
+	run_on_all ../edit-contents deep/testfile &&
+	test_all_match git add deep/testfile &&
+	run_on_all ../edit-contents deep/testfile &&
+
+	test_all_match git diff &&
+	test_all_match git diff --staged &&
+
+	# Add file outside cone
+	test_all_match git reset --hard &&
+	run_on_all mkdir newdirectory &&
+	run_on_all ../edit-contents newdirectory/testfile &&
+	test_all_match git sparse-checkout set newdirectory &&
+	test_all_match git add newdirectory/testfile &&
+	run_on_all ../edit-contents newdirectory/testfile &&
+	test_all_match git sparse-checkout set &&
+
+	test_all_match git diff &&
+	test_all_match git diff --staged &&
+
+	# Merge conflict outside cone
+	test_all_match git reset --hard &&
+	test_all_match git checkout merge-left &&
+	test_all_match test_must_fail git merge merge-right &&
+
+	test_all_match git diff &&
+	test_all_match git diff --staged
+'
+
 # NEEDSWORK: sparse-checkout behaves differently from full-checkout when
 # running this test with 'df-conflict-2' after 'df-conflict-1'.
 test_expect_success 'diff with renames and conflicts' '
@@ -1429,6 +1466,11 @@ test_expect_success 'sparse-index is not expanded' '
 	ensure_not_expanded reset --merge update-deep &&
 	ensure_not_expanded reset --hard &&
 
+	echo a test change >>sparse-index/README.md &&
+	ensure_not_expanded diff &&
+	git -C sparse-index add README.md &&
+	ensure_not_expanded diff --staged &&
+
 	ensure_not_expanded reset base -- deep/a &&
 	ensure_not_expanded reset base -- nonexistent-file &&
 	ensure_not_expanded reset deepest -- deep &&