perms: make '-c' required for batch mode

perms: make '-c' required for batch mode

quoth Tony:

>   Typing ^C at a command to make it stop without doing anything is a natural
>   reaction, regardless of any imprecations to type "cancel"

He asked for a '-b' option; i.e., don't allow a bare "ssh git@... perms repo"
to be interpreted as batch mode.

Meanwhile, '-c' is acceptable for existing repos too (as you can see from the
mirroring code at least).

So, just think of '-c' as a moral eqvt of '-b' that already exists.