add: don't complain when adding empty project root

This behavior was added in 07d7bedd (add: don't complain when adding
empty project root - 2009-04-28) then broken by 84b8b5d1 (remove
match_pathspec() in favor of match_pathspec_depth() -
2013-07-14). Reinstate it.
Noticed-by: NThomas Ferris Nicolaisen <tfnico@gmail.com>
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -544,7 +544,7 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 
 		for (i = 0; i < pathspec.nr; i++) {
 			const char *path = pathspec.items[i].match;
-			if (!seen[i] &&
+			if (!seen[i] && path[0] &&
 			    ((pathspec.items[i].magic &
 			      (PATHSPEC_GLOB | PATHSPEC_ICASE)) ||
 			     !file_exists(path))) {

t/t3700-add.sh

@@ -272,6 +272,25 @@ test_expect_success '"add non-existent" should fail' '
 	! (git ls-files | grep "non-existent")
 '
 
+test_expect_success 'git add -A on empty repo does not error out' '
+	rm -fr empty &&
+	git init empty &&
+	(
+		cd empty &&
+		git add -A . &&
+		git add -A
+	)
+'
+
+test_expect_success '"git add ." in empty repo' '
+	rm -fr empty &&
+	git init empty &&
+	(
+		cd empty &&
+		git add .
+	)
+'
+
 test_expect_success 'git add --dry-run of existing changed file' "
 	echo new >>track-this &&
 	git add --dry-run track-this >actual 2>&1 &&