D672: merge: add `ondisk` to merge.update and pass it in largefiles

[phillco (Phil Cohen)]

phillco added a comment.


  Abandoned in favor of the approach in https://phab.mercurial-scm.org/D682.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D672

To: phillco, #hg-reviewers
Cc: mercurial-devel