merge: add _checkunknowndirs function for detecting path conflicts Add a new function which, given a file name, finds the shortest path for which there is a conflicting file or directory in the working directory. Differential Revision: https://phab.mercurial-scm.org/D779

merge: backup conflicting directories when getting files During batchget, if a target file conflicts with a directory, or if the directory a target file is in conflicts with a file, backup and remove the conflicting file or directory before performing the get. Differential Revision: https://phab.mercurial-scm.org/D778