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
running: init test1
result: 0
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: 0
running: commit -m commit2 -d 2000-01-02 foo
result: 0
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: 0
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: 0