Add --check option

[JoaquimEsteves]

✨ New argument: --check

As the name indicates this will merely check if the code would have
been re-written.

A further change is that the return value of blacken-docs is now the
largest error for each file. Previously if file 2 failed with an error
but file 1 would be re-written then the exit code would have been 1 as
opposed to the more severe 2.

Closes #277

[adamchainz]

Please add tests and a changelog note.

[adamchainz]

Please write a description of the option rather than just link and say "it's like that". The link is still useful though, but don’t link a whole sentence, just a couple words.

Also alphabetize by moving to line 119.

[JoaquimEsteves]

Should be fixed in f01c5ab

Do tell me if you're unhappy with the phrasing.

[adamchainz]

Let’s match the formatting of the existing message.

Suggested change

	print(f"Error: Would have been re-written - {filename}")
	print(f"{filename}: Would rewrite.")

[JoaquimEsteves]

Done in: f01c5ab

I did use a different error message:

print(f"{filename}: Requires a rewrite.")

I feel like it reads better; but if you're unhappy I can always just do the one you suggest.

[JoaquimEsteves]

Sorry for the delay; was on holiday.

I also added the tests and the changelog.

From context I think that the changelog requires a version bump; I did so in the setup.cfg but do warn me if I've done it wrong.

[JoaquimEsteves]

Dear @adamchainz,

Anything else you'd like to see before merging?

Sorry to be a bother; but since it's been a while so I decided to ping

[pfouque]

Anything else you'd like to see before merging?

You should rollback the version number bump and remove the release date and version from the changelog (and also bring back the bitwise operator if the usage of max is not required)
I can't commit, but it will be easier to merge once it's done.
Thanks

[pfouque]

This change is usually decided by the release manager depending on the changelog at release time: see example

[pfouque]

It seems this line is added at release time usually (visible on this release PR)

Suggested change

	1.17.0 (2023-09-11)
	-------------------

[adamchainz]

Thanks for your patience, I just reviewed and fixed up your PR. Looking good now, going to merge. Thanks for your contribution!