diff-files: integrate with sparse index

Remove full index requirement for `git diff-files`
and test to ensure the index is not 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
-----------------------------------------------------------------
2000.78: git diff-files (full-v3)              0.09    0.08 -11.1%
2000.79: git diff-files (full-v4)              0.09    0.09 +0.0%
2000.80: git diff-files (sparse-v3)            0.52    0.02 -96.2%
2000.81: git diff-files (sparse-v4)            0.51    0.02 -96.1%
2000.82: git diff-files f2/f4/a (full-v3)      0.06    0.07 +16.7%
2000.83: git diff-files f2/f4/a (full-v4)      0.08    0.08 +0.0%
2000.84: git diff-files f2/f4/a (sparse-v3)    0.46    0.01 -97.8%
2000.85: git diff-files f2/f4/a (sparse-v4)    0.51    0.02 -96.1%

Signed-off-by: Shuqi Liang <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Cheskaqiqi

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

@@ -125,5 +125,7 @@ test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
 test_perf_on_all "git rm -f $SPARSE_CONE/a && git checkout HEAD -- $SPARSE_CONE/a"
 test_perf_on_all git grep --cached --sparse bogus -- "f2/f1/f1/*"
+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

@@ -2103,4 +2103,17 @@ test_expect_success 'diff-files with pathspec outside sparse definition' '
 	test_cmp expect sparse-checkout-out
 '
 
+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
+'
+
 test_done