Minor improvements on code readability

Adding comments on a non-intuitive behavior ar `_format_str_once`, and
improving the `test_one_empty_line` test readability by using a
pre-defined function that implements the same assertions

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

src/black/__init__.py

@@ -1107,6 +1107,8 @@ def _format_str_once(src_contents: str, *, mode: Mode) -> str:
     for block in dst_blocks:
         dst_contents.extend(block.all_lines())
     if mode.preview and not dst_contents:
+        # Use decode_bytes to retrieve the correct source newline (CRLF or LF),
+        # and check if normalized_content has more than one line
         normalized_content, _, newline = decode_bytes(src_contents.encode("utf-8"))
         if "\n" in normalized_content:
             return newline

tests/test_black.py

@@ -159,10 +159,7 @@ def test_one_empty_line(self) -> None:
         mode = black.Mode(preview=True)
         for nl in ["\n", "\r\n"]:
             source = expected = nl
-            actual = fs(source, mode=mode)
-            self.assertFormatEqual(expected, actual)
-            black.assert_equivalent(source, actual)
-            black.assert_stable(source, actual, mode)
+            assert_format(source, expected, mode=mode)
 
     def test_one_empty_line_ff(self) -> None:
         mode = black.Mode(preview=True)