Add negation to merge files, adds CACHEDIR.TAG support

[kaspar030]

This PR makes the previously forbidden ! modifier available to merge and dir-merge filters.
It's effect is that it turns the default for that filter file to - /** (which is added after the merge file rules).

Additionally, unless the e modifier is also set, another rule for including the rules file is added.

This by itself makes it possible to honour CACHEDIR.TAG files by specifying --filter='dir-merge,+! CACHEDIR.TAG'. The CACHEDIR.TAG is still parsed. Its first line is usually Signature: 8a477f597d28d172789f06886806bc55, so currently a rule for that is added, but it would in practice never(tm) match.

I could imagine adding another modifier specifying a merge file is just a tag, meaning it is not parsed at all. Maybe t. Or, special-case the CACHEDIR.TAG filename. Or, ignore all rules with the exact content Signature: 8a477f597d28d172789f06886806bc55. But also this is already very usable as is, and IMO a pretty small change.

Additionally it would be easy to add aliases like tar and others have them:

       --exclude-caches
              Exclude contents of directories containing file CACHEDIR.TAG,
              except for the tag file itself. -> --filter='dir-merge,+! CACHEDIR.TAG'

       --exclude-caches-all
              Exclude directories containing file CACHEDIR.TAG and the file
              itself. -> --filter='dir-merge,+e! CACHEDIR.TAG'

       --exclude-caches-under
              Exclude everything under directories containing CACHEDIR.TAG

Let me know what you think!

Implements #451, #499.
For reference, the CACHEDIR.TAG spec: https://bford.info/cachedir/