Missing --jobs support

[overratedpro]

Monitoring the resource consumption with multiple Python files staged in a pre-commit scenario, the CPU is not used very effectively, since only a single subprocess is invoked. There should be a way to parallelize the workload over separate files, utilizing all available CPU cores to achieve better performance.

[rogalski]

Let's rename this issue into "missing --jobs support"

[rogalski]

Please note that despite we initially suspected CPU bottleneck, the actual root cause is different, see #204 #205 #206

[akaihola]

If we manage to improve bisection performance and short-circuit bisection on Black failures, do you still see --jobs necessary? If so, can you live without it until a 1.4.0 release a few weeks later?