Comprehensions should not split between tuple-unpacking commas

`split_all_top_level_comma_separated_values` inserts spurious breaks between commas in tuple-unpacking expressions in comprehensions.

Hand-formatted code:

```
results = {
field_name: func(field_value)
for field_name, field_value in self.fields.items()
}
```

YAPF changes this to:

```
results = {
field_name: func(field_value)
for field_name,
field_value in self.fields.items()
}
```

The third line is short enough to fit within the line limit without breaking, and putting a break there decreases readability. I can't see any logical reason there should be a break there.

If I turn off `split_all_top_level_comma_separated_values`, YAPF does not change my hand-formatted code.

Full configuration:

```
[style]
based_on_style = pep8
column_limit = 88
dedent_closing_brackets = False
spaces_before_comment = 2
split_all_top_level_comma_separated_values = True
split_before_arithmetic_operator = True
split_before_logical_operator = True
split_complex_comprehension = True
```