Add fish shell completions support (#686).

Merge pull request #70 from isaec/feat/fish-completions

tests/integration/commands/test_completion.py

@@ -3,3 +3,7 @@ from borgmatic.commands import completion as module
 
 def test_bash_completion_does_not_raise():
     assert module.bash_completion()
+
+
+def test_fish_completion_does_not_raise():
+    assert module.fish_completion()