Perform the first gc.collect before clearing cache

In issue gitpython-developers#553 it was originally mentioned that the gc.collect was
required before clearing the cache, but in the code the collect happens
after the cache clear. Move the first gc.collect to before the cache to
be consistent with the original description.

Author: emonty

Diff for: git/repo/base.py

@@ -199,9 +199,10 @@ def __del__(self):
 
     def close(self):
         if self.git:
-            self.git.clear_cache()
             if self._gc_on_close:
                 gc.collect()
+            self.git.clear_cache()
+            if self._gc_on_close:
                 gitdb.util.mman.collect()
                 gc.collect()