Extract function SetLocalIndentWidth(num_spaces)

Makes the code shorter and simpler.

This does sometimes set tabstop or softtabstop when it’s unnecessary to do so (because expandtab is the wrong value), but there is no harm in doing that. And in fact it is helpful, because it reduces the work the user has to do if they manually change the value of expandtab – the option will already be at the correct value.
Reword and reorder my fork differences section in README
Set 0 for `shiftwidth` and -1 for `softtabstop` when supported

The version checks are separated into separate functions to make their purpose clearer, and to provide places for the citation of patch numbers in comments.

This is another reason why I consolidated indent option setting into a single function earlier.
Add pointer to look at docs in usage comments

I feel like it might be a bad practice to duplicate the docs in the function comments, so I don’t want to redescribe the function options. But maybe there was a good reason for putting the comments there. So I compromise by instructing the developer to look at the docs.

At least now the usage comments are clearly labeled incomplete, so a developer won’t think that the two options mentioned are the only ones.
Pluralize “refactoring” in README

I *think* this is more grammatical