pattern matching, docs vs. reality

While dealing with #7631, we noted that this patterns file does not work as I expected:
```
P sh
R /Users/tw/w/borg
+ Users/tw/w/borg/src
+ Users/tw/w/borg/src/borg
+ Users/tw/w/borg/src/borg/helpers
+ Users/tw/w/borg/src/borg/helpers/**
- **
```
The idea was to include the parent dir fs items (**only** the 3 directory items `src`, `borg` and `helpers`, not the files inside), then include the desired files (here: all inside helpers) and exclude everything else.

Issue: It included **all** below `Users/tw/w/borg/src` into the backup archive (not just the dir items and the files in helpers).

So it seems not to work as I thought it would (I personally do my backups the other way: include all, only exclude what I do not want).

Update:

This is already enough to get that behaviour:
```
P sh
R /Users/tw/w/borg
+ Users/tw/w/borg/src
- **
```

Reproduced with borg 1.2.3, 1.2.4 and 1.2-maint git branch.