fix status messages

Since implementing the source mirroring, we have been seeing the git messages which check for submodules and such that are supposed to be silent, back in the logs.

This fixes the Source to ensure that we are silencing the messages while accessing Source.get_source_fetchers(), and also adds an explicit message to git's fetch implementation, stating the URL it is fetching from, which consequently also silences the nested messages which occur at fetch time.

buildstream/_platform/linux.py

@@ -55,6 +55,10 @@ class Linux(Platform):
             return SandboxBwrap(*args, **kwargs)
 
     def check_sandbox_config(self, config):
+        if not self._local_sandbox_available():
+            # Accept all sandbox configs as it's irrelevant with the dummy sandbox (no Sandbox.run).
+            return True
+
         if self._user_ns_available:
             # User namespace support allows arbitrary build UID/GID settings.
             return True