exclude_pattern with 1 glob starting with ./ doesn't match any files (work in progress)

The exclusion filter doesn't find any files when given a single glob starting with ./ (related to #1695). Since no files are found, it returns an empty array and does not remove the files from the set to be run. As a result, files that we would expect to be excluded are not excluded.

__Pattern starts with spec/__
```
rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb  --exclude-pattern spec/features/\*\*/\*_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
 1/1 |========================= 100 ==========================>| Time: 00:00:00

1 example, 0 failures
```

__Pattern starts with ./spec/__
```
rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb  --exclude-pattern ./spec/features/\*\*/\*_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
 3/3 |========================= 100 ==========================>| Time: 00:00:01

3 examples, 0 failures
```

I should have noticed this when I fixed the inclusion case. :(  I'll take a shot at this soon; it should be easy having just fixed the other case.
Actually, could this use `RSpec::Core::Metadata.relative_path`?