Fix ReferenceFileSystem.cat for unsorted lists of paths

[ap--]

Hello everyone,

here's a potential fix for the issue revealed in zarr-developers/zarr-python#1324

I added a test to check the order of returned data when using FSMap.getitems
Without the fix, it errors with

    def test_mapping_getitems(m):
        m.pipe({"a": b"A", "b": b"B"})
    
        refs = {
            "a": ["a"],
            "b": ["b"],
        }
        h = fsspec.filesystem("memory")
        fs = fsspec.filesystem("reference", fo=refs, fs=h)
        mapping = fs.get_mapper("")
>       assert mapping.getitems(["b", "a"]) == {"a": b"A", "b": b"B"}
E       AssertionError: assert {'a': b'B', 'b': b'A'} == {'a': b'A', 'b': b'B'}
E         Differing items:
E         {'b': b'A'} != {'b': b'B'}
E         {'a': b'B'} != {'a': b'A'}
E         Full diff:
E         - {'a': b'A', 'b': b'B'}
E         + {'a': b'B', 'b': b'A'}

Regarding the fix: I'm not sure if this should be solved differently, since the non-merging codepath is probably the more common. Also I haven't tested if DFReferenceFileSystem.cat has the same issue...

Cheers,
Andreas 😃

[martindurant]

Don't worry about DFReferenceFileSystem, it is strictly experimental.