Add filter_file option

[intosi]

Add filter_file option. The value gets passed directly to rsync using the
--filter='merge /path/to/filter/file' directive.

Test-Information:

Unit tests pass.
Tested extensively on my local backup machine.

[bebehei]

Thanks for the PR.

The code in rsnapshot-program.pl looks good. There had been no need to add much logic for filter-file. The rsnapshot.1 has to get removed out of the changeset and probably some tests are missing. Besides this, the PR looks good.

Now my personal opinion: TBH this is a bit of overengineering rsnapshot. I've always seen rsnapshot as a simplification of rsync to do backups and this PR is giving rsnapshot more complexity. Many people don't even understand rsyncs exclude-arguments as you can see on rsnapshot-discuss. I'm not sure yet, which way to go.

I'll let the PR open and decide later which way to go.

[nkadel]

filterfile can be really, useful to update the configurations out of band with the basic rsnapshot configuration. It can also make a much more detailed and refined inclusion or exclusion list than --exclude or --include, due to the maximum line length limitations of most operating systems.

[MTecknology]

@bebehei The changes seem relatively trivial and clean. I don't imagine a large number of people would use this feature, but I can think of a few scenarios where it'd be useful. The people I picture using it /should/ know how that option works within rsync. Is there any chance you could weigh in your opinion as of today?

@intosi (depending on the response) Is there any chance you could take a look at resolving the merge conflicts and also add a few tests?