log: changed implementation to use graphlog code

Now that revsets work in a lazy way, log code can be changed to parse every
option into a revset and then evaluate it lazily.

Now expressions like

  "hg log -b default -b ."

are converted into a revset using the same code as graphlog.

--HG--
extra : amend_source : 77f93efd26273d44dbf3dc541fbd476d6b482a31
merge: with merge.preferancestor=*, run an auction with bids from ancestors

The basic idea is to do the merge planning with all the available ancestors,
consider the resulting actions as "bids", make an "auction" and
automatically pick the most favourable action for each file.

This implements the basic functionality and will only consider "keep" and
"get" actions. The heuristics for picking the best action can be tweaked later
on.

By default it will only pass ctx.ancestor as the single ancestor to
calculateupdates. The code path for merging with a single ancestor is not
changed.