merge: improve error messages for path conflicts during update Differential Revision: https://phab.mercurial-scm.org/D782

merge: check for path conflicts when updating (issue5628) When updating to a new revision, check for path conflicts caused by unknown files in the working directory, and handle these by backing up the file or directory and replacing it. Differential Revision: https://phab.mercurial-scm.org/D781

merge: rename conflicts to fileconflicts in _checkunknownfiles We will need to distinguish between file conflicts and path conflicts. Rename the conflicts variable so that it will be clearly distinct from pathconflicts, which will be introduced in a future commit. Differential Revision: https://phab.mercurial-scm.org/D780