revset: add startdepth limit to ancestors() as internal option
This is necessary to implement the set{gen} (set subscript) operator. For
example, set{-n} will be translated to ancestors(set, depth=n, startdepth=n).
https://www.mercurial-scm.org/wiki/RevsetOperatorPlan#ideas_from_mpm
The UI is undecided and I doubt if the startdepth option would be actually
useful, so the option is hidden for now. 'depth' could be extended to take
min:max range, in which case, integer depth should select a single generation.
ancestors(set, depth=:y) # scan up to y-th generation
ancestors(set, depth=x:) # skip until (x-1)-th generation
ancestors(set, depth=x) # select only x-th generation
Any ideas are welcomed.
# reverse(ancestors(tip)) using hg repo
3) 0.075951
4) 0.076175
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