`max_width` is off by one for use statements

[lmmx]

I was surprised to notice that rustfmt is off by one when it comes to handling use item identifiers, from what I can see.

It is not explicitly stated in the Configurations.md file but the max_width (default: 100) is expected to behave as <= but in practice it operates as <. Lines with use identifier lists of length 100 are wrapped, those at 99 are not.

I note there are other issues in a similar vein like #6180, but this doesn't seem to have been reported.

I used wc -L here to show the max line length.