make --exclude-if-present support shell-like patterns

Yes, I checked docs, FAQ and open issues.
Borg version: 1.1.10 on Fedora F31.

This is an enhancement request.

It would be nice if `--exclude-if-present PATTERN` was understood and not just `--exclude-if-present NAME`. That way
```
borg create --keep-exclude-tags --exclude-if-present sh:/path/**/.git /path ...
```
would backup everything under /path but if a git repository was present in any directory then only that repo and nothing else from the directory. Best also except if another subdirectory again has a .git submodule that subdirectory then again is backed up.