Allow disabling parallel mode from the command line

[neutrinoceros]

Is your feature request related to a problem? Please describe.
I frequently encounter projects that set parallel=true in coveragerc or equivalent, supposedly because they only run coverage through CI, where it makes sense. I find disabling parallel mode is useful for local runs;
if I don't turn it off somehow, coverage report fails with No data to report.

Currently, there isn't a way (as far as I can tell) to override this flag from the command line, so I just end up making temporary changes to the config file, that I must remember not to commit, creating friction. I also frequently forget about this workaround and find myself re-discovering it.

Describe the solution you'd like
Having some way to override the flag from the command line (e.g. --parallel-mode=off or --no-parallel-mode) would help.

Describe alternatives you've considered

• coverage report and friends (coverage html...) could support parallel=true and lookup data where it actually is. This seems like the truly ideal solution but I don't have a clear view of the big picture so maybe there are technical/historical reasons why this is not feasible ?
• The error message emitted by coverage report and friends could be improved by hinting at parallel-mode being activated and/or that data exists, just not in the default .coverage directory.

Additional context
I'm willing to implement any solution that you deem suitable myself. Though I suspect that changing the CLI might be potentially tricky to me since I've never used optparse.

[nedbat]

The error message emitted by coverage report and friends could be improved by hinting at parallel-mode being activated and/or that data exists, just not in the default .coverage directory.

As it happens, #1781 and #1782 are working on this.

[neutrinoceros]

oh wow, great timing, thanks !

[nedbat]

I'll mark this as a dup of #1781.