hg: rewrite "copystore()" with vfs
This patch rewrites "copystore()" with vfs, because succeeding patch
requires "lock.lock()" invocation with vfs.
This patch uses 'dstbase + "/lock"' instead of "join()" with both
elements even on Windows environment but it should be reasonable,
because target files given from "store.copyfiles()" already uses "/"
as path separator.
"util.copyfiles()" between two vfs-s may have to be rewritten in the
future.