Add hook for ensuring vcs permalinks

tests/check_vcs_permalinks_test.py

@@ -0,0 +1,36 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+from pre_commit_hooks.check_vcs_permalinks import main
+
+
+def test_trivial(tmpdir):
+    f = tmpdir.join('f.txt').ensure()
+    assert not main((f.strpath,))
+
+
+def test_passing(tmpdir):
+    f = tmpdir.join('f.txt')
+    f.write_binary(
+        # permalinks are ok
+        b'https://github.com/asottile/test/blob/649e6/foo%20bar#L1\n'
+        # links to files but not line numbers are ok
+        b'https://github.com/asottile/test/blob/master/foo%20bar\n',
+    )
+    assert not main((f.strpath,))
+
+
+def test_failing(tmpdir, capsys):
+    with tmpdir.as_cwd():
+        tmpdir.join('f.txt').write_binary(
+            b'https://github.com/asottile/test/blob/master/foo#L1\n',
+        )
+
+        assert main(('f.txt',))
+        out, _ = capsys.readouterr()
+        assert out == (
+            'f.txt:1:https://github.com/asottile/test/blob/master/foo#L1\n'
+            '\n'
+            'Non-permanent github link detected.\n'
+            'On any page on github press [y] to load a permalink.\n'
+        )