Add Bash completion script so you can tab-complete the borgmatic command-line.

tests/end-to-end/test_completion.py

@@ -0,0 +1,5 @@
+import subprocess
+
+
+def test_bash_completion_runs_without_error():
+    subprocess.check_call('eval "$(borgmatic --bash-completion)"', shell=True)

tests/integration/commands/test_completion.py

@@ -0,0 +1,5 @@
+from borgmatic.commands import completion as module
+
+
+def test_bash_completion_does_not_raise():
+    assert module.bash_completion()