grep: turn off gitlink detection for --no-index

If we are running "git grep --no-index" outside of a git
repository, we behave roughly like "grep -r", examining all
files in the current directory and its subdirectories.
However, because we use fill_directory() to do the
recursion, it will skip over any directories which look like
sub-repositories.

For a normal git operation (like "git grep" in a repository)
this makes sense; we do not want to cross the boundary out
of our current repository into a submodule. But for
"--no-index" without a repository, we should look at all
files, including embedded repositories.

There is one exception, though: we probably should _not_
descend into ".git" directories. Doing so is inefficient and
unlikely to turn up useful hits.

This patch drops our use of dir.c's gitlink-detection, but
we do still avoid ".git". That makes us more like tools such
as "ack" or "ag", which also know to avoid cruft in .git.

As a bonus, this also drops our usage of the ref code
when we are outside of a repository, making the transition
to pluggable ref backends cleaner.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -522,12 +522,14 @@ static int grep_objects(struct grep_opt *opt, const struct pathspec *pathspec,
 }
 
 static int grep_directory(struct grep_opt *opt, const struct pathspec *pathspec,
-			  int exc_std)
+			  int exc_std, int use_index)
 {
 	struct dir_struct dir;
 	int i, hit = 0;
 
 	memset(&dir, 0, sizeof(dir));
+	if (!use_index)
+		dir.flags |= DIR_NO_GITLINKS;
 	if (exc_std)
 		setup_standard_excludes(&dir);
 
@@ -902,7 +904,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		int use_exclude = (opt_exclude < 0) ? use_index : !!opt_exclude;
 		if (list.nr)
 			die(_("--no-index or --untracked cannot be used with revs."));
-		hit = grep_directory(&opt, &pathspec, use_exclude);
+		hit = grep_directory(&opt, &pathspec, use_exclude, use_index);
 	} else if (0 <= opt_exclude) {
 		die(_("--[no-]exclude-standard cannot be used for tracked contents."));
 	} else if (!list.nr) {

t/t7810-grep.sh

@@ -905,6 +905,33 @@ test_expect_success 'inside git repository but with --no-index' '
 	)
 '
 
+test_expect_success 'grep --no-index descends into repos, but not .git' '
+	rm -fr non &&
+	mkdir -p non/git &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+
+		echo magic >file &&
+		git init repo &&
+		(
+			cd repo &&
+			echo magic >file &&
+			git add file &&
+			git commit -m foo &&
+			echo magic >.git/file
+		) &&
+
+		cat >expect <<-\EOF &&
+		file
+		repo/file
+		EOF
+		git grep -l --no-index magic >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'setup double-dash tests' '
 cat >double-dash <<EOF &&
 --