vfs: add "readlines" and "tryreadlines" This patch allows "readlines" and "tryreadlines" to take "mode" argument, because "subrepo" requires to read files not in "rb" (binary, default for vfs) but in "r" (text) mode in subsequent patch.

subrepo: add "_cachestorehashvfs" to handle cache store hash files via vfs This "vfs" object will be used by subsequent patches to handle cache store hash files without direct file APIs. This patch decorates "_cachestorehashvfs" with "@propertycache" to delay vfs creation, because it is used only for cooperation with other repositories. In this patch, "/" is used as the path separator, even though "self._repo.join" uses platform specific path separator (e.g. "\\" on Windows). But it is reasonable enough, because "store" and other management file handling already include such implementation, and they work well.