grep: grep cache entries if they are "assume unchanged"

"Assume unchanged" bit means "please pretend that I have never touched
this file", so  if user removes the file, we should not care.

This patch teaches "git grep" to use cache version in such
situations. External grep case has not been fixed yet. But given that
on the platform that CE_VALID bit may be used like Windows, external
grep is not available anyway, I would wait for people to raise their
hands before touching it.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -404,7 +404,12 @@ static int grep_cache(struct grep_opt *opt, const char **paths, int cached)
 			continue;
 		if (!pathspec_matches(paths, ce->name))
 			continue;
-		if (cached) {
+		/*
+		 * If CE_VALID is on, we assume worktree file and its cache entry
+		 * are identical, even if worktree file has been modified, so use
+		 * cache version instead
+		 */
+		if (cached || (ce->ce_flags & CE_VALID)) {
 			if (ce_stage(ce))
 				continue;
 			hit |= grep_sha1(opt, ce->sha1, ce->name, 0);

t/t7002-grep.sh

@@ -161,7 +161,14 @@ test_expect_success 'log grep (6)' '
 	git log --author=-0700  --pretty=tformat:%s >actual &&
 	>expect &&
 	test_cmp expect actual
+'
 
+test_expect_success 'grep with CE_VALID file' '
+	git update-index --assume-unchanged t/t &&
+	rm t/t &&
+	test "$(git grep --no-ext-grep t)" = "t/t:test" &&
+	git update-index --no-assume-unchanged t/t &&
+	git checkout t/t
 '
 
 test_done