[perf] Add caching and lock protections

tests/test_context_processors.py

@@ -4,9 +4,15 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from vrobbler import context_processors
 from vrobbler.context_processors import version_info
 
 
+@pytest.fixture(autouse=True)
+def reset_git_cache():
+    context_processors._GIT_COMMIT = None
+
+
 @pytest.fixture
 def mock_request():
     return MagicMock()