--check option, mirroring black --check

[akaihola]

--check skips writing changes to files, and returns a return value of 1 from the darker process in case any modified lines in any of the given files are not already properly formatted.

The return value will allow using darker with more different tools. It for example opens the possibility for a pytest-darker plugin.

[akaihola]

I rebased on master and resolved conflicts.

[samoylovfp]

Looks great!

A couple of aesthetic nitpicks of questionable usefulness:

• the call format_edited_parts(but_dont_really_edit_the_files=True) is a little wonky IMO.
• the fact that the parameter print_diff implies check_only is a little weird to me as well.

I would refactor the code so there is a function
that returns the proposed changes
and another function that actually applies those,
so the high-level control remains high in the call stack and the "do_the_actual_change" flag does not have to travel far in the function calls.

[akaihola]

@samoylovfp, both --diff and --check do imply that changes aren't written into files. The reason for this is simply that it's how Black works. This ensures that Darker can more easily act as a direct replacement for Black when integrating with other tools.

[akaihola]

I would refactor the code so there is a function
that returns the proposed changes
and another function that actually applies those

I did that, could you make a another quick check on the code? Quite some test changes were required as a result.

[samoylovfp]

@akaihola I think it looks excellent!