There are too much code in Mercurial that assume file function from extension.
Keeping the original extension when doing backup is just creating confusion
(for exemple, backup of revlog's confused with new version revlogs).

So we change the backup name from now on to be explicilty backup file and
prevent this kind of problem.

(note that repository using fncache is less affected (but still affected) by
this kind of problem as the backup are not listed in the fncache).