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.
The idea is being able to associate a file with a property, and watch
that file stat info for modifications when we decide it's important for it to
be up-to-date. Once it changes, we recreate the object.
On filesystems that can't uniquely identify a file, we always recreate.
As a consequence, localrepo.invalidate() will become much less expensive in the
case where nothing changed on-disk.
