log: remove increasing windows usage in fastpath
The purpose of increasing windows is to allow backwards iteration on the
filelog at a reasonable cost.
But is it needed?
- if follow is False, we have no reason to iterate backwards.
We basically just want to walk the complete filelog and yield all revisions
within the revision range. We can do this forward or
backwards, as it only reads the index.
- when follow is True, we need to examine the contents of the filelog, and to
do this efficiently we need to read the filelog forward.
And on the other hand, to track ancestors and copies, we need to process
revisions backwards. But is it necessary to use increasing windows
for this?
We can iterate over the complete filelog forward, stack the revisions, and
read the reversed(pile), it does the same thing with a more readable code.