Fix a hidden bug which makes argument list grow longer for ever.

The old code updates the argument list when it is asked to refresh
process info, but it calls ProcessInfo::addArgument() without ever
clearing existing arguements first. This means the argument list will
grow longer and longer for ever. For a long-running shell session in
which user has executed thousands of commands, that will waste quite
amount of memory, not to metion the logic error itself.

The reaseon why this bug keeps hidden is that the %c and %C formatter
have never been revealed to users, which are not implemented yet.