atomictempfile: make close() consistent with other file-like objects. The usual contract is that close() makes your writes permanent, so atomictempfile's use of close() to *discard* writes (and rename() to keep them) is rather unexpected. Thus, change it so close() makes things permanent and add a new discard() method to throw them away. discard() is only used internally, in __del__(), to ensure that writes are discarded when an atomictempfile object goes out of scope. I audited mercurial.*, hgext.*, and ~80 third-party extensions, and found no one using the existing semantics of close() to discard writes, so this should be safe.