fix: check for multiprocessing in a --concurrency list - #2270

coverage run refuses run-affecting command-line options alongside --concurrency=multiprocessing, since they cannot reach the subprocesses. The check compares the whole option value with ==, so it never fires for a comma list, and the run proceeds with the parent measuring branch coverage and the subprocesses not:

$ coverage run --concurrency=multiprocessing,thread --branch prog.py
(exit 0, no message)
$ coverage combine
Can't combine statement coverage data with branch data

after:
$ coverage run --concurrency=multiprocessing,thread --branch prog.py
Options affecting multiprocessing must only be specified in a configuration file.
Remove --branch from the command line.
(exit 1)

The comma form is not exotic, doc/migrating.rst tells people to write it: "Now that must be explicitly enabled with --concurrency=multiprocessing,thread". The single value form was refused correctly the whole time.

The fix splits on commas, which is what cmdline.py already does with the same string a few hundred lines up when it builds the Coverage object, so the two now agree on what the option means.

I checked that --concurrency=thread --branch still exits 0, that multiprocessing,thread with no extra flags still exits 0, and that thread,multiprocessing --branch is refused as well.

AI-assisted: written with Claude Code, reviewed and tested by me before sending.