grep: ignore --recurse-submodules if --no-index is given

Since grep learned to recurse into submodules in 0281e487fd
(grep: optionally recurse into submodules, 2016-12-16),
using --recurse-submodules along with --no-index makes Git
die().

This is unfortunate because if submodule.recurse is set in a user's
~/.gitconfig, invoking `git grep --no-index` either inside or outside
a Git repository results in

    fatal: option not supported with --recurse-submodules

Let's allow using these options together, so that setting submodule.recurse
globally does not prevent using `git grep --no-index`.

Using `--recurse-submodules` should not have any effect if `--no-index`
is used inside a repository, as Git will recurse into the checked out
submodule directories just like into regular directories.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -96,7 +96,8 @@ OPTIONS
 	Recursively search in each submodule that has been initialized and
 	checked out in the repository.  When used in combination with the
 	<tree> option the prefix of all submodule output will be the name of
-	the parent project's <tree> object.
+	the parent project's <tree> object. This option has no effect
+	if `--no-index` is given.
 
 -a::
 --text::

builtin/grep.c

@@ -958,6 +958,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			/* die the same way as if we did it at the beginning */
 			setup_git_directory();
 	}
+	/* Ignore --recurse-submodules if --no-index is given or implied */
+	if (!use_index)
+		recurse_submodules = 0;
 
 	/*
 	 * skip a -- separator; we know it cannot be
@@ -1115,8 +1118,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (recurse_submodules && (!use_index || untracked))
-		die(_("option not supported with --recurse-submodules"));
+	if (recurse_submodules && untracked)
+		die(_("--untracked not supported with --recurse-submodules"));
 
 	if (!show_in_pager && !opt.status_only)
 		setup_pager();

t/t7814-grep-recurse-submodules.sh

@@ -345,7 +345,16 @@ test_incompatible_with_recurse_submodules ()
 }
 
 test_incompatible_with_recurse_submodules --untracked
-test_incompatible_with_recurse_submodules --no-index
+
+test_expect_success 'grep --recurse-submodules --no-index ignores --recurse-submodules' '
+	git grep --recurse-submodules --no-index -e "^(.|.)[\d]" >actual &&
+	cat >expect <<-\EOF &&
+	a:(1|2)d(3|4)
+	submodule/a:(1|2)d(3|4)
+	submodule/sub/a:(1|2)d(3|4)
+	EOF
+	test_cmp expect actual
+'
 
 test_expect_success 'grep --recurse-submodules should pass the pattern type along' '
 	# Fixed