file-lines do not reformat parts of a multi-line statement

Imagine the following statement in which one line is wrongly indented:

let diff = repo
.diff_tree_to_workdir(Some(&head), Some(&mut diff_options))
    .unwrap();

I now specify only the second line in my --file-lines.

What I expect to happen
The second line becomes correctly indented to match the same column as the .unwrap().

What actually happens
Nothing. The line is ignored because it is part of a larger statement.

Why this is a problem
I have a use case where a script uses git and --file-lines to only format lines which where changed in a specific commit. This script fails when someone just edits parts of a larger statement because the line is not formatted then. Adding more context to the range is also not viable because then lines are formatted that weren't touched in the commit.

#3397

Reproduce the example with rustfmt and the --file-lines option, using the multi-line statement shown in the issue. Trace how selected lines are handled when they belong to a larger statement; done means the selected line is reformatted without formatting untouched lines in the same statement.