store: cleanup what is recognized as a revlog file The temporary file from `censor` are not relevant to the store as they are just temporary. So let `fncache` and `datafiles` forget about them.

store: only inclure revlog related file in `datafiles` This is a "reading" equivalent of the previous patches (see that patch for details).

store: do not record file that are not part of a revlog in fncache The fncache exist to list file to copy/stream when cloning a repository, it should only contains file that are relevant for a revlog in such case. For example, temporary file are not relevant. So we now skip the addiction of non-relevant file to the fn-cache in the first place.

revlog-split: expand the some test to cover the success case too It seems useful to double check the case of a transaction successfully committed before trying the one with transaction abort.

transaction: use a ".bck" extension for all backup file 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).

repair-issue6528: remove a now useless continue The condition for that continue have been excplicly filtered out of the `files` list a handful of line earlier.