diff-files: integrate with sparse index

Remove full index requirement for `git diff-files`. Refactor the
ensure_expanded and ensure_not_expanded functions by introducing a
common helper function, ensure_index_state. Add test to ensure the index
is no expanded in `git diff-files`.

The `p2000` tests demonstrate a ~96% execution time reduction for 'git
diff-files' and a ~97% execution time reduction for 'git diff-files'
for a file using a sparse index:

Test                                               before  after
-----------------------------------------------------------------------
2000.94: git diff-files (full-v3)                  0.09    0.08 -11.1%
2000.95: git diff-files (full-v4)                  0.09    0.09 +0.0%
2000.96: git diff-files (sparse-v3)                0.52    0.02 -96.2%
2000.97: git diff-files (sparse-v4)                0.51    0.02 -96.1%
2000.98: git diff-files -- f2/f4/a (full-v3)       0.06    0.07 +16.7%
2000.99: git diff-files -- f2/f4/a (full-v4)       0.08    0.08 +0.0%
2000.100: git diff-files -- f2/f4/a (sparse-v3)    0.46    0.01 -97.8%
2000.101: git diff-files -- f2/f4/a (sparse-v4)    0.51    0.02 -96.1%

Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-files.c

@@ -27,6 +27,10 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 		usage(diff_files_usage);
 
 	git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
+
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	repo_init_revisions(the_repository, &rev, prefix);
 	rev.abbrev = 0;
 

t/perf/p2000-sparse-operations.sh

@@ -129,5 +129,7 @@ test_perf_on_all git grep --cached bogus -- "f2/f1/f1/*"
 test_perf_on_all git write-tree
 test_perf_on_all git describe --dirty
 test_perf_on_all 'echo >>new && git describe --dirty'
+test_perf_on_all git diff-files
+test_perf_on_all git diff-files -- $SPARSE_CONE/a
 
 test_done

t/t1092-sparse-checkout-compatibility.sh

@@ -1377,7 +1377,7 @@ test_expect_success 'index.sparse disabled inline uses full index' '
 	! test_region index ensure_full_index trace2.txt
 '
 
-ensure_not_expanded () {
+run_sparse_index_trace2 () {
 	rm -f trace2.txt &&
 	if test -z "$WITHOUT_UNTRACKED_TXT"
 	then
@@ -1397,7 +1397,16 @@ ensure_not_expanded () {
 			git -C sparse-index "$@" \
 			>sparse-index-out \
 			2>sparse-index-error || return 1
-	fi &&
+	fi
+}
+
+ensure_expanded () {
+	run_sparse_index_trace2 "$@" &&
+	test_region index ensure_full_index trace2.txt
+}
+
+ensure_not_expanded () {
+	run_sparse_index_trace2 "$@" &&
 	test_region ! index ensure_full_index trace2.txt
 }
 
@@ -2147,4 +2156,18 @@ test_expect_success 'diff-files with pathspec outside sparse definition' '
 	test_all_match git diff-files -- "folder*/a"
 '
 
+test_expect_success 'sparse index is not expanded: diff-files' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	run_on_all ../edit-contents deep/a &&
+
+	ensure_not_expanded diff-files &&
+	ensure_not_expanded diff-files -- deep/a &&
+	ensure_not_expanded diff-files -- "deep/*"
+'
+
 test_done