When clearing unknown files to remove path conflicts, also delete files that
conflict with the target file's path.
Differential Revision: https://phab.mercurial-scm.org/D785
When merging, check for any path conflicts introduced by the manifest
merge and rename the conflicting file to a safe name.
Differential Revision: https://phab.mercurial-scm.org/D784
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
