context: tell when .ancestor picks one of multiple common ancestors heads

Show a message like
  note: using 0f6b37dbe527 as ancestor of adfe50279922 and cf89f02107e5

So far this is just a warning - there is nothing the user can do to select
another ancestor.
context: introduce merge.preferancestor for controlling which ancestor to pick

Multiple revisions can be specified in merge.preferancestor, separated by
whitespace. First match wins.

This makes it possible to overrule the default of picking the common ancestor
with the lowest hash value among the "best" (introduced in 3605d4e7e618).

This can for instance help with some merges where the 'wrong' ancestor is used.
There will thus be some overlap between this and the problems that can be
solved with a future 'consensus merge'.

Mercurial will show a note like
  note: using 40663881a6dd as ancestor of 3b08d01b0ab5 and adfe50279922
        alternatively, use --config merge.preferancestor=0f6b37dbe527
when the option is available, listing all the alternative ancestors.