merge_imports will incorrectly format imports with self

[nagisa]

$ cat /tmp/test.rs 
use foo::{self};
$ rustfmt /tmp/test.rs --emit=stdout --config-path rustfmt.toml
/tmp/test.rs:

use foo::self;

The newly formatted code cannot compile because Rust requires self to appear within {}. Related: #3807.

[calebcartwright]

Yup, indeed a bug. Will open a PR with a fix shortly