subrepo: use vfs.walk instead of os.walk...
subrepo: use vfs.walk instead of os.walk
"dirpath" in the tuple yielded by "vfs.walk()" is relative one from
the root of specified vfs, and absolute path in the warning message is
composed by "vfs.join()".
On the other hand, target file "f" exists in "dirpath", and
"reljoin()" is needed to unlink "f" by "vfs.unlink()".
