grep: fix worktree case in submodules

Running git-grep with --recurse-submodules results in a cached grep for
the submodules even when --cached is not used. This makes all
modifications in submodules' tracked files be always ignored when
grepping. Solve that making git-grep respect the cached option when
invoking grep_cache() inside grep_submodule(). Also, add tests to
ensure that the desired behavior is performed.

Reported-by: Daniel Zaoui <jackdanielz@eyomi.org>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/grep.c b/builtin/grep.c
index 580fd38..467eb45 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -403,7 +403,7 @@ static int grep_tree(struct grep_opt *opt, const struct pathspec *pathspec,
 static int grep_submodule(struct grep_opt *opt,
 			  const struct pathspec *pathspec,
 			  const struct object_id *oid,
-			  const char *filename, const char *path)
+			  const char *filename, const char *path, int cached)
 {
 	struct repository subrepo;
 	struct repository *superproject = opt->repo;
@@ -474,7 +474,7 @@ static int grep_submodule(struct grep_opt *opt,
 		strbuf_release(&base);
 		free(data);
 	} else {
-		hit = grep_cache(&subopt, pathspec, 1);
+		hit = grep_cache(&subopt, pathspec, cached);
 	}
 
 	repo_clear(&subrepo);
@@ -522,7 +522,8 @@ static int grep_cache(struct grep_opt *opt,
 			}
 		} else if (recurse_submodules && S_ISGITLINK(ce->ce_mode) &&
 			   submodule_path_match(repo->index, pathspec, name.buf, NULL)) {
-			hit |= grep_submodule(opt, pathspec, NULL, ce->name, ce->name);
+			hit |= grep_submodule(opt, pathspec, NULL, ce->name,
+					      ce->name, cached);
 		} else {
 			continue;
 		}
@@ -597,7 +598,8 @@ static int grep_tree(struct grep_opt *opt, const struct pathspec *pathspec,
 			free(data);
 		} else if (recurse_submodules && S_ISGITLINK(entry.mode)) {
 			hit |= grep_submodule(opt, pathspec, &entry.oid,
-					      base->buf, base->buf + tn_len);
+					      base->buf, base->buf + tn_len,
+					      1); /* ignored */
 		}
 
 		strbuf_setlen(base, old_baselen);