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()".
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.