restore cursor to same line of code, not same line of buffer (#989)

plugin/black.vim

@@ -110,18 +110,42 @@ def Black():
     string_normalization=not bool(int(vim.eval("g:black_skip_string_normalization"))),
     is_pyi=vim.current.buffer.name.endswith('.pyi'),
   )
-  buffer_str = '\n'.join(vim.current.buffer) + '\n'
+  (cursor_line, cursor_column) = vim.current.window.cursor
+  cb = vim.current.buffer[:]
+  cb_bc = cb[0:cursor_line]
+  # Format all code before the cursor.
+  # Detect unclosed blocks, close them with pass.
+  last_line = cb_bc[-1]
+  if last_line.rstrip().endswith(":"):
+      cb_bc[-1] = last_line + " pass"
+  # Determine old:new cursor location mapping
+  buffer_str_before = '\n'.join(cb_bc)+'\n'
+  try:
+    new_buffer_str_before = black.format_file_contents(buffer_str_before, fast=fast, mode=mode)
+    new_cb = new_buffer_str_before.split('\n')[:-1]
+    new_cursor_line = len(new_cb)
+    new_cursor = (new_cursor_line, cursor_column)
+  except black.NothingChanged:
+    new_cursor_line = cursor_line
+    new_cursor = (new_cursor_line, cursor_column)
+  except Exception as exc:
+    print(exc)
+  # Now we know where the cursor should be
+  # when we format the entire buffer. Do it:
+  buffer_str = '\n'.join(cb) + '\n'
   try:
     new_buffer_str = black.format_file_contents(buffer_str, fast=fast, mode=mode)
   except black.NothingChanged:
     print(f'Already well formatted, good job. (took {time.time() - start:.4f}s)')
   except Exception as exc:
     print(exc)
   else:
-    cursor = vim.current.window.cursor
-    vim.current.buffer[:] = new_buffer_str.split('\n')[:-1]
+    # Replace the buffer
+    new_cb = new_buffer_str.split('\n')[:-1]
+    vim.current.buffer[:] = new_cb
+    # Restore the cursor to its rightful place
     try:
-      vim.current.window.cursor = cursor
+      vim.current.window.cursor = new_cursor
     except vim.error:
       vim.current.window.cursor = (len(vim.current.buffer), 0)
     print(f'Reformatted in {time.time() - start:.4f}s.')