do not skip "--reference" if package-meta ...

... is enabled and git's partial clone is used.

This should mainly be relevant for servers/containers running with
caching enabled (export CACHEDIRECTORY=....)

Fixes: #495

TarSCM/scm/git.py

@@ -421,7 +421,7 @@ def prepare_working_copy(self):
         use_reference = True
 
         try:
-            if self.args.package_meta:
+            if (self.args.package_meta and not self.partial_clone):
                 logging.info("Not using '--reference'")
                 use_reference = False
         except KeyError: