Fix unstable format involving backslash + whitespace at beginning of file (#948)

black.py

@@ -1480,7 +1480,13 @@ def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         lines (two on module-level).
         """
         before, after = self._maybe_empty_lines(current_line)
-        before -= self.previous_after
+        before = (
+            # Black should not insert empty lines at the beginning
+            # of the file
+            0
+            if self.previous_line is None
+            else before - self.previous_after
+        )
         self.previous_after = after
         self.previous_line = current_line
         return before, after

tests/data/beginning_backslash.py

@@ -0,0 +1,12 @@
+\
+
+
+
+
+
+print("hello, world")
+
+# output
+
+
+print("hello, world")

tests/test_black.py

@@ -639,6 +639,14 @@ def test_tuple_assign(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_beginning_backslash(self) -> None:
+        source, expected = read_data("beginning_backslash")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     def test_tab_comment_indentation(self) -> None:
         contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t# comment\n\tpass\n"
         contents_spc = "if 1:\n    if 2:\n        pass\n    # comment\n    pass\n"