Merge branch 'cc/subprocess-handshake-missing-capabilities' into maint

When handshake with a subprocess filter notices that the process asked for an unknown capability, Git did not report what program the offending subprocess was running. This has been corrected. We may want a follow-up fix to tighten the error checking, though. * cc/subprocess-handshake-missing-capabilities: sub-process: print the cmd when a capability is unsupported