Group imports together with imports_granularity results in non-idempotent formatting

When group_imports = "StdExternalCrate" is combined with imports_granularity = "Module", rustfmt can format a file in a non-idempotent way, i.e. running rustfmt twice in a row results in a different formatting.

Reduced from a case in the stdlib (https://github.com/rust-lang/rust/issues/126394).

Minimal repro:

# rustfmt.toml
version = "Two"
group_imports = "StdExternalCrate"
imports_granularity = "Module"

$ rustfmt --version
rustfmt 1.7.0-nightly (72fdf91 2024-06-05)

# file.rs
use a::c;
// foo
use a::b;
use a::d;

First run (rustfmt file.rs):

// foo
use a::b;
use a::{c, d};

Second run (rustfmt file.rs):

// foo
use a::{b, c, d};

Removing the group_imports or imports_granularity options removes the issue. The problem seems to be caused by the comment, without it it works fine.

Start with the minimal rustfmt.toml and file.rs reproduction, running rustfmt twice with group_imports set to StdExternalCrate and imports_granularity set to Module. Trace the import-grouping and comment-handling entry points involved in the changed output. Done means formatting the reproduction is idempotent while preserving the reported behavior without either option.