Подвижный: Слияние одного файла между ответвлениями в одном repo

Nope. Mercurial works on a changeset basis.

But you can do a "dummy merge" where you ignore the incoming changes from one of the branches. Before you commit you could then revert selected files to whatever state you want:

% HGMERGE=internal:local hg merge     # keep my files
% hg revert --rev other-branch a.txt  # update a.txt to other branch
% hg commit -m 'Dummy merge to pick a.txt from other-branch.'

Maybe that will help you a bit.