test-largefiles: demonstrate problems with renaming and reverting a directory These things were uncovered looking at issue5738. First, if the destination directory exists under .hglf, the source is moved under the destination instead of renaming the last component for `hg mv srcdir dstdir`. This is extra confusing, because it occurs even if the user visible destination (i.e. the path _not_ under .hglf) does not exist. Additionally, when a largefile is forgotten via revert, any modifications end up getting clobbered. For normal files, the forgotten file is left unchanged, as shown by test-import.t. The forget command on a largefile will correctly leave the file unmodified.