Sparse index: git diff

This change enables running the `diff` builtin command without
expanding the full index in a cone-mode sparse checkout. It adds basic
infrastructure to "light up" sparse index for this command.

Additionally, it includes tests to verify sparse index is working as
expected. These tests ensure that:

1. The index is not expanded for `diff` and `diff --staged` (which both
involve reading from the index).
2. `diff` and `diff --staged` behave the same in full
checkout, sparse checkout, and 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. Merge conflict for paths outside sparse-checkout cone.

Author: ldennington

builtin/diff.c

@@ -437,6 +437,9 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 
 	prefix = setup_git_directory_gently(&nongit);
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	if (!no_index) {
 		/*
 		 * Treat git diff with at least one path outside of the

t/t1092-sparse-checkout-compatibility.sh

@@ -387,6 +387,45 @@ test_expect_success 'diff --staged' '
 	test_all_match git diff --staged
 '
 
+test_expect_success 'diff partially-staged' '
+	init_repos &&
+
+	write_script add-file-in-cone <<-\EOF &&
+	git sparse-checkout set deep
+	echo >deep/testfile
+	git add deep/testfile
+	echo a new line >>deep/testfile
+	EOF
+	run_on_all ../add-file-in-cone &&
+
+	test_all_match git diff &&
+	test_all_match git diff --staged &&
+
+	write_script add-file-outside-cone <<-\EOF &&
+	git reset --hard
+	mkdir newdirectory
+	echo >newdirectory/testfile
+	git sparse-checkout set newdirectory
+	git add newdirectory/testfile
+	echo a new line >>newdirectory/testfile
+	git sparse-checkout set
+	EOF
+	run_on_all ../add-file-outside-cone &&
+
+	test_all_match git diff &&
+	test_all_match git diff --staged &&
+
+	write_script merge-conflict-outside-cone <<-\EOF &&
+	git reset --hard
+	git checkout merge-left
+	EOF
+	run_on_all ../merge-conflict-outside-cone &&
+
+	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' '
@@ -822,6 +861,12 @@ test_expect_success 'sparse-index is not expanded' '
 	ensure_not_expanded reset base -- folder1 &&
 
 	ensure_not_expanded reset --hard update-deep &&
+
+	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 checkout -f update-deep &&
 	(
 		sane_unset GIT_TEST_MERGE_ALGORITHM &&