evolvecmd: _possibledestination() skip visiting empty parent
I see that even in the case when "parent == -1" we are visiting
that also and find succsets for that and in last -1 is being
added in the set of possible destination. Why not skip that?
pick: `hg help` was not showing the full cmd desc of `pick`
after running command `hg help` it was not including the second line
of command description. I guess it picks the firstline only. So I moved
that part in first line of cmd description.