Phase out get_dataset_root()

[mih]

Right now this is only used in the tree command. It uses the datalad-core implementation that has a complex behavior (involving normpathing). At the same time, this is little more than

git rev-parse --show-toplevel

[mih]

Like with #631 the rev-parse approach is too slow to justify the reduction in code complexity

Getting the equivalent of ../../../..:

>>> %timeit get_dataset_root('.')
19 µs ± 40.1 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

>>> %timeit call_git_oneline(['rev-parse', '--show-toplevel'])
2.26 ms ± 9.78 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)