add: warn when asked to update SKIP_WORKTREE entries

`git add` already refrains from updating SKIP_WORKTREE entries, but it
silently exits with zero code when it is asked to do so. Instead, let's
warn the user and display a hint on how to update these entries.

Note that we only warn the user whey they give a pathspec item that
matches no eligible path for updating, but it does match one or more
SKIP_WORKTREE entries. A warning was chosen over erroring out right away
to reproduce the same behavior `add` already exhibits with ignored
files. This also allow users to continue their workflow without having
to invoke `add` again with only the eligible paths (as those will have
already been added).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3705-add-sparse-checkout.sh

@@ -36,10 +36,26 @@ setup_gitignore () {
 	EOF
 }
 
+test_expect_success 'setup' "
+	cat >sparse_error_header <<-EOF &&
+	The following pathspecs didn't match any eligible path, but they do match index
+	entries outside the current sparse checkout:
+	EOF
+
+	cat >sparse_hint <<-EOF &&
+	hint: Disable or modify the sparsity rules if you intend to update such entries.
+	hint: Disable this message with \"git config advice.updateSparsePath false\"
+	EOF
+
+	echo sparse_entry | cat sparse_error_header - >sparse_entry_error &&
+	cat sparse_entry_error sparse_hint >error_and_hint
+"
+
 test_expect_success 'git add does not remove sparse entries' '
 	setup_sparse_entry &&
 	rm sparse_entry &&
-	git add sparse_entry &&
+	test_must_fail git add sparse_entry 2>stderr &&
+	test_cmp error_and_hint stderr &&
 	test_sparse_entry_unchanged
 '
 
@@ -47,7 +63,8 @@ test_expect_success 'git add -A does not remove sparse entries' '
 	setup_sparse_entry &&
 	rm sparse_entry &&
 	setup_gitignore &&
-	git add -A &&
+	git add -A 2>stderr &&
+	test_must_be_empty stderr &&
 	test_sparse_entry_unchanged
 '
 
@@ -55,7 +72,13 @@ test_expect_success 'git add . does not remove sparse entries' '
 	setup_sparse_entry &&
 	rm sparse_entry &&
 	setup_gitignore &&
-	git add . &&
+	test_must_fail git add . 2>stderr &&
+
+	cat sparse_error_header >expect &&
+	echo . >>expect &&
+	cat sparse_hint >>expect &&
+
+	test_cmp expect stderr &&
 	test_sparse_entry_unchanged
 '
 
@@ -64,7 +87,8 @@ do
 	test_expect_success "git add${opt:+ $opt} does not update sparse entries" '
 		setup_sparse_entry &&
 		echo modified >sparse_entry &&
-		git add $opt sparse_entry &&
+		test_must_fail git add $opt sparse_entry 2>stderr &&
+		test_cmp error_and_hint stderr &&
 		test_sparse_entry_unchanged
 	'
 done
@@ -73,14 +97,16 @@ test_expect_success 'git add --refresh does not update sparse entries' '
 	setup_sparse_entry &&
 	git ls-files --debug sparse_entry | grep mtime >before &&
 	test-tool chmtime -60 sparse_entry &&
-	git add --refresh sparse_entry &&
+	test_must_fail git add --refresh sparse_entry 2>stderr &&
+	test_cmp error_and_hint stderr &&
 	git ls-files --debug sparse_entry | grep mtime >after &&
 	test_cmp before after
 '
 
 test_expect_success 'git add --chmod does not update sparse entries' '
 	setup_sparse_entry &&
-	git add --chmod=+x sparse_entry &&
+	test_must_fail git add --chmod=+x sparse_entry 2>stderr &&
+	test_cmp error_and_hint stderr &&
 	test_sparse_entry_unchanged &&
 	! test -x sparse_entry
 '
@@ -89,8 +115,41 @@ test_expect_success 'git add --renormalize does not update sparse entries' '
 	test_config core.autocrlf false &&
 	setup_sparse_entry "LINEONE\r\nLINETWO\r\n" &&
 	echo "sparse_entry text=auto" >.gitattributes &&
-	git add --renormalize sparse_entry &&
+	test_must_fail git add --renormalize sparse_entry 2>stderr &&
+	test_cmp error_and_hint stderr &&
 	test_sparse_entry_unchanged
 '
 
+test_expect_success 'git add --dry-run --ignore-missing warn on sparse path' '
+	setup_sparse_entry &&
+	rm sparse_entry &&
+	test_must_fail git add --dry-run --ignore-missing sparse_entry 2>stderr &&
+	test_cmp error_and_hint stderr &&
+	test_sparse_entry_unchanged
+'
+
+test_expect_success 'do not advice about sparse entries when they do not match the pathspec' '
+	setup_sparse_entry &&
+	test_must_fail git add nonexistent 2>stderr &&
+	grep "fatal: pathspec .nonexistent. did not match any files" stderr &&
+	! grep -F -f sparse_error_header stderr
+'
+
+test_expect_success 'do not warn when pathspec matches dense entries' '
+	setup_sparse_entry &&
+	echo modified >sparse_entry &&
+	>dense_entry &&
+	git add "*_entry" 2>stderr &&
+	test_must_be_empty stderr &&
+	test_sparse_entry_unchanged &&
+	git ls-files --error-unmatch dense_entry
+'
+
+test_expect_success 'add obeys advice.updateSparsePath' '
+	setup_sparse_entry &&
+	test_must_fail git -c advice.updateSparsePath=false add sparse_entry 2>stderr &&
+	test_cmp sparse_entry_error stderr
+
+'
+
 test_done