Fix printing of indented block comments

[janmasrovira]

• Closes Repeated reformatting of multi-line comments resulting in them moving to the right #3143

The formatting of block comments has changed slightly.
A comment block has the syntax {- multi-line-text -}.
The rules are:

1. The first line, i.e. the line that starts with {- is trimmed of trailing space.
2. The last line, i.e. the line that ends with -} is trimmed of prefix space.
3. Lines in the middle are trimmed of prefix and trailing space.

This makes it impossible to have such a comment:

{-
  indented
-}

The reasoning is that if we don't want to have an indentation sensitive parser we have two choices.

1. Print all comments flushed to the left
2. Do not have indentation in the middle lines.

I thought 2. was better