sparse-checkout: document interactions with submodules

Using 'git submodule (init|deinit)' a user can select a subset of
submodules to populate. This behaves very similar to the sparse-checkout
feature, but those directories contain their own .git directory
including an object database and ref space. To have the sparse-checkout
file also determine if those files should exist would easily cause
problems. Therefore, keeping these features independent in this way
is the best way forward.

Also create a test that demonstrates this behavior to make sure
it doesn't change as the sparse-checkout feature evolves.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-sparse-checkout.txt

@@ -164,6 +164,16 @@ case-insensitive check. This corrects for case mismatched filenames in the
 'git sparse-checkout set' command to reflect the expected cone in the working
 directory.
 
+
+SUBMODULES
+----------
+
+If your repository contains one or more submodules, then those submodules will
+appear based on which you initialized with the `git submodule` command. If
+your sparse-checkout patterns exclude an initialized submodule, then that
+submodule will still appear in your working directory.
+
+
 SEE ALSO
 --------
 

t/t1091-sparse-checkout-builtin.sh

@@ -340,4 +340,32 @@ test_expect_success 'cone mode: set with core.ignoreCase=true' '
 	test_cmp expect dir
 '
 
+test_expect_success 'interaction with submodules' '
+	git clone repo super &&
+	(
+		cd super &&
+		mkdir modules &&
+		git submodule add ../repo modules/child &&
+		git add . &&
+		git commit -m "add submodule" &&
+		git sparse-checkout init --cone &&
+		git sparse-checkout set folder1
+	) &&
+	list_files super >dir &&
+	cat >expect <<-\EOF &&
+		a
+		folder1
+		modules
+	EOF
+	test_cmp expect dir &&
+	list_files super/modules/child >dir &&
+	cat >expect <<-\EOF &&
+		a
+		deep
+		folder1
+		folder2
+	EOF
+	test_cmp expect dir
+'
+
 test_done