hg-core: add a compilation error if trying to compile outside of Linux
For now, we can only provide support for Linux in `hg-core`, so we have to be
explicit about it in case anyone wonders why their Dirstate is suddenly broken,
or why the crate does not compile (on Windows for example).
Differential Revision: https://phab.mercurial-scm.org/D8246
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.