vfs: define "join()" in each classes derived from "abstractvfs" This patch defines "join()" in each classes derived from "abstractvfs" except "vfs", which already defines it. This allows all vfs instances to be used for indirect file API invocation.

store: initialize vfs field first to use it for initialization of others This patch initializes "vfs" field in the constructor of each store classes to use it for initialization of others. In this patch, "self.vfs.base" is used to initialize "self.path", because redo join of path components for "self.path" is redundant.

scmutil: reorder newly added functions for vfs support in dictionary order Definition functions for vfs support in dictionary order increases readability/maintainability, because there are functions which invoke file API: - with same name: "os.listdir" and "osutil.listdir", for example - with ambiguous names: "os.mkdir" and "util.makedirs", for example