grep: Fix -f FILE when FILE is empty and -x provided

Grep currently special-cased empty pattern file to be the same as
pattern file with one empty line (empty pattern). That does mirror how
GNU grep behaves, except when -x is provided. In that case .* pattern
needs to be used instead.

Signed-off-by: Gray Wolf <wolf@wolfsden.cz>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/grep.tests

@@ -129,6 +129,12 @@ testing "grep -v -f EMPTY_FILE" \
 	"" \
 	"test\n"
 
+testing "grep -vxf EMPTY_FILE" \
+	"grep -vxf input" \
+	"test\n" \
+	"" \
+	"test\n"
+
 testing "grep -Fw matches only words" \
 	"grep -Fw foo input" \
 	"" \