Don't double-indent multiline expressions in binary operations

[reese]

Now that binary expressions are modeled as breakables, they (correctly) add an indentation level when they split to multiple lines. However, we usually don't actually start that indentation until after the initial LHS expression, so we need to account for this and dedent the LHS to prevent double-indenting.

Before:

{
  a: Foo
      .calculate_stuff / # Too deep!
    2
}

After:

{
  a: Foo
    .calculate_stuff / # Lookin' good
    2
}