Get data_file path from Coverage config in subprocesses.

Fix #145.

Author: jezdez

Diff for: src/pytest_cov/engine.py

@@ -40,7 +40,8 @@ def set_env(self):
             os.environ['COV_CORE_CONFIG'] = config_file
         else:
             os.environ['COV_CORE_CONFIG'] = ''
-        os.environ['COV_CORE_DATAFILE'] = os.path.abspath('.coverage')
+        data_file = os.path.abspath('.coverage')
+        os.environ['COV_CORE_DATAFILE'] = getattr(self.config, 'data_file', data_file)
         if self.cov_branch:
             os.environ['COV_CORE_BRANCH'] = 'enabled'