util: add safename function for generating safe names to rename to This function finds a name which does not clash with any other name in the manifest, and so can be used to safely rename a file. Differential Revision: https://phab.mercurial-scm.org/D783

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