hg: use "os.path.join()" to join path components which may be empty (issue4203) Changset 2d0ab571b822 rewriting "hg.copystore()" with vfs uses 'dstbase + "/lock"' instead of "os.path.join()", because target files given from "store.copyfiles()" already uses "/" as path separator But in the repository using revlog format 0, "dstbase" becomes empty ("data" directory is located under ".hg" directly), and 'dstbase + "/lock"' is treated as "/lock": in almost all cases, write access to "/lock" causes "permission denied". This patch uses "os.path.join()" to join path components which may be empty in "hg.copystore()".