A type mismatch caused the search for the other head to fail. The code is
fragile, and instead it ended up using the 'first' bookmark head, but the
ordering is undefined and it could thus randomly use the wrong bookmarkhead
and fail with:
$ hg up -q -C e@diverged
$ hg merge
abort: merging with a working directory ancestor has no effect
Bookmarks will behave more like named branches when merge tries to pick
a revision to merge.
Bookmarks now to respect the current bookmarks. Bookmarks will not
accidentally merged with unnamed heads or other bookmarks. However merge
can pick heads with diverging bookmarks and pick those automatically.
We end up with two cases for picking a revision to merge:
(1) In case of an current bookmark, merge can pick a branch head that has a
diverged bookmark
(2) In case of no current bookmark, merge can pick a branch head that does not
have a bookmark.
