wrap_comments on/off have inconsistent trailing whitespace handling in code blocks

[cjhopman]

With wrap_comments = false, rustfmt enforces that an empty line at the beginning of a code block has no trailing whitespace. With wrap_comments = true, rustfmt enforces taht that same line has a single trailing space.

The formatting of wrap_comments=true should probably be an accepted formatting with wrap_comments=false.

Repro:

[0s]: buck$ cat rustfmt.toml
edition = "2018"
merge_imports = true
merge_derives = true
use_field_init_shorthand = true
use_try_shorthand = true
wrap_comments = true
[0s]: buck$ cat fmt.rs | tr " " "#"
//!#```
//!
//!#use#something;
//!#```
[0s]: buck$ rustfmt fmt.rs
[0s]: buck$ cat fmt.rs | tr " " "#"
//!#```
//!#
//!#use#something;
//!#```
[0s]: buck$ vim rustfmt.toml
[2s]: buck$ cat rustfmt.toml
edition = "2018"
merge_imports = true
merge_derives = true
use_field_init_shorthand = true
use_try_shorthand = true
[0s]: buck$ rustfmt --check fmt.rs
Diff in /Users/cjhopman/buck/fbcode/buck/fmt.rs at line 1:
 //! ```
-//!
+//!
 //! use something;
 //! ```