normalize_comments closing brace indentation bug with hard_tabs

[stbuehler]

Hi,

with the following config:

hard_tabs = true
normalize_comments = true

the snippet:

/// ```
/// Data {
///     a: "some text data",
///     ..Default::default()
/// };
/// ```
#[derive(Default)]
pub struct Data {
	a: &str,
	b: u32,
}

gets formatted as:

/// ```
/// Data {
/// 	a: "some text data",
/// 	..Default::default()
/// 	};
/// ```
#[derive(Default)]
pub struct Data {
	a: &str,
	b: u32,
}

Please note the (wrongly) indented closing brace in the doc code block. With hard_tabs = false it works fine.

Tested with rustfmt 0.99.6-nightly (750b252 2018-10-18).

I'd also appreciate it if there was a separate comment_hard_tabs setting (or it simply would always be false), as I think tabs should only be at the beginning of a line, and not within comments - if you're open to it I can open another issue.