`--update-data` introduces new blank lines when aligning multi-line expected info comments

[wyattscarpenter]

Bug Report

After using --update-data in #19904, I'm seeing a lot of diffs like this:

 [case testErrorCodeIgnoreMultiple1]
 a = 'x'.foobar(b)  # type: ignore[name-defined, attr-defined]
 a = 'x'.foobar(b)  # type: ignore[name-defined, xyz]  # E: "str" has no attribute "foobar"  [attr-defined] \
-    # N: Error code "attr-defined" not covered by "type: ignore" comment
+                                                      # N: Error code "attr-defined" not covered by "type: ignore" comment ignoring [name-defined, xyz]
 a = 'x'.foobar(b)  # type: ignore[xyz, w, attr-defined]  # E: Name "b" is not defined  [name-defined] \
-    # N: Error code "name-defined" not covered by "type: ignore" comment
+                                                         # N: Error code "name-defined" not covered by "type: ignore" comment ignoring [xyz, w, attr-defined]
+
+

it seems to apply extra whitespace at the end of the test case if it realigns a line continuation

Semi-related to #17465, but only a little, I think.

[wyattscarpenter]

The error is probably in here:

mypy/mypy/test/update_data.py

Line 53 in 19697af

# Update assertion comments within the sections

In a reverse of the normal course of events, I have the test case figured out already... it turns out the normal test was using backslashes wrong, so fixing that creates a test for this which is already failing. wyattscarpenter@9018224 (I will also add a test case for alignment specifically, later.)