When computing the DAG depth, we walk through all ancestors: this commit adds
memoization during that first step. Then, the memorized parents are fetched
from a dict instead of calling parents() on each vertex.
This tweak, according to Kcachegrind, improves ancestor() performance by 16%
when cloning a big repository.
Make depth calculation non-recursive
Add simple shortcut for linear ancestry
Convert context to use ancestor function
make memoized parents function
Convert revlog to use ancestor function
