Currently, it is implemented on top of commonancestorsheads.
Implement it on top of reachableroots instead, as reachableroots could
stop walking the graph much sooner than commonancestorsheads.
Measuring repo.changelog.isancestorrev on two revisions in a private repository:
before: ! wall 0.005175 comb 0.010000 user 0.010000 sys 0.000000 (best of 550)
after : ! wall 0.000072 comb 0.000000 user 0.000000 sys 0.000000 (best of 36199)
When hg does this kind of operations 1500 times in pull ->
bookmarks.comparebookmarks -> bookmarks.validdest, that's 11s that
drop from the --profile output.