revset: add ^ and ~ operators from parentrevspec extension
^ (Nth parent) and ~ (Nth first ancestor) are infix operators that match
certain ancestors of the set:
set^0
the set
set^1 (also available as set^)
the first parent of every changeset in set
set^2
the second parent of every changeset in set
set~0
the set
set~1
the first ancestor (i.e. the first parent) of every changeset in set
set~2
the second ancestor (i.e. first parent of first parent) of every changeset
in set
set~N
the Nth ancestor (following first parents only) of every changeset in set;
set~N is equivalent to set^1^1..., with ^1 repeated N times.