churn: ignore trailing and leading spaces (issue2546)

merge with stable

make_file: always return a fresh file handle that can be closed Currently, cmdutil.make_file() will return a freshly made file handle, except when given a pattern of '-'. If callers would want to close the handle, they would have to make sure that it's neither sys.stdin or sys.stdout. Instead, returning a duplicate of either of the two ensures that make_file() lives up to its name and creates a new file handle regardless of the input.