Assure index file descriptor is closed after reader (#1394) (#1395)

A regression that was introduced with d79d20d.

Author: Byron

git/index/base.py

@@ -135,7 +135,10 @@ def _set_cache_(self, attr: str) -> None:
                 return None
             # END exception handling
 
-            stream = file_contents_ro(fd, stream=True, allow_mmap=True)
+            try:
+                stream = file_contents_ro(fd, stream=True, allow_mmap=True)
+            finally:
+                os.close(fd)
 
             self._deserialize(stream)
         else: