This removes use of unknown files for building the synthetic working
directory manifest used by manifestmerge. Instead, we adopt the
strategy used by _checkunknown.
Side-effect: unknown files are no longer moved by remote directory
renames, and now are left alone like ignored files.
Globbing is usually used for filenames, so on windows it is reasonable and very
convenient that glob patterns accepts '\' or '/' when the pattern specifies
'/'.
