Add Buffer::n_lines()

test/src/test_Buffer.cc

@@ -33,11 +33,13 @@ TEST(BufferTest, writes_an_empty_file_for_an_empty_buffer)
     Buffer b;
     ASSERT_TRUE(b.write_to_file("test/tmp/f"));
     TestSupport::compare_files("test/files/empty.txt", "test/tmp/f");
+    ASSERT_EQ(0u, b.n_lines());
 }
 
 TEST(BufferTest, allows_navigating_using_iterators)
 {
     Buffer b("test/files/line_endings/lf_format.txt");
+    ASSERT_EQ(2u, b.n_lines());
     auto iterator = b.add_cursor();
 
     EXPECT_EQ(0u, iterator->line());