revlog: speed up isancestor
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.
Differential Revision: https://phab.mercurial-scm.org/D6506