opener: use posixfile to hold file open when calling nlinks() Mercurial's posixfile is less intrusive on Windows than Python's open

remove pointless os.path.join calls when opening files in .hg/cache Windows deals just fine with '/' in paths and Mercurial on Windows already does file accesses elsewhere with mixed \ and / in file paths anyway. This patch also makes the static-http repo case use proper URLs of the form http://example.com/repo/.hg/cache/branchheads http://example.com/repo/.hg/cache/tags instead of the entirely pointless http://example.com/repo/.hg/cache%5Cbranchheads http://example.com/repo/.hg/cache%5tags (as introduced by 5ccdca7df211)