diff: ensure consistent diff behavior with ignore options

In git-diff, options like `-w` and `-I<regex>`, two files are considered
equivalent under the specified "ignore" rules, even when they are not
bit-for-bit identical. For options like `--raw`, `--name-status`,
and `--name-only`, git-diff deliberately compares only the SHA values
to determine whether two files are equivalent, for performance reasons.
As a result, a file shown in `git diff --name-status` may not appear
in `git diff --patch`.

To quickly determine whether two files are equivalent, add a helper
function diff_flush_patch_quietly() in diff.c. Add `.dry_run` field in
`struct diff_options`. When `.dry_run` is true, builtin_diff() returns
immediately upon finding any change. Call diff_flush_patch_quietly()
to determine if we should flush `--raw`, `--name-only` or `--name-status`
output.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -400,6 +400,8 @@ struct diff_options {
 	#define COLOR_MOVED_WS_ERROR (1<<0)
 	unsigned color_moved_ws_handling;
 
+	bool dry_run;
+
 	struct repository *repo;
 	struct strmap *additional_path_headers;