kbknapp / clap-rs / 3093

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Auto merge of #832 - mgeisler:fix-wrapping-bugs, r=kbknapp

Fix wrapping bugs

I've been working towards integrating my [textwrap][1] crate and along the way, I found some small problems in the existing `wrap_help` function in clap. I basically added new code that calls both `textwrap::fill` and `wrap_help` and panicked on any difference:
```
fn wrap_help(help: &mut String, longest_w: usize, avail_chars: usize) {
    let input = help.clone();
    let mut old = help.clone();
    old_wrap_help(&mut old, longest_w, avail_chars);

    let mut wrapped = String::with_capacity(help.len());
    for (i, line) in help.lines().enumerate() {
        if i > 0 {
            wrapped.push('\n');
        }
        wrapped.push_str(&textwrap::fill(line, avail_chars));
    }

    // TODO: move up, This keeps old behavior of not wrapping at all
    // if one of the words would overflow the line
    if longest_w < avail_chars {
        *help = wrapped;
    }

    if *old != *help {
        println!("********************************");
        println!("longest_w: {}, avail_chars: {}", longest_w, avail_chars);
        println!("help: {:3} bytes: {:?}", input.len(), input);
        println!("old:  {:3} bytes: {:?}", old.len(), old);
        println!("new:  {:3} bytes: {:?}", help.len(), help);
        println!("********************************");
        panic!("bad wrap");
    }
}

fn old_wrap_help(help: &mut String, longest_w: usize, avail_chars: usize) {
    // ... as before
```

This PR fixes two small problems discovered this way, one of which became #828.

[1]: https://crates.io/crates/textwrap

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