Close .git/index file descriptor

Author: skshetry

Diff for: git/index/base.py

@@ -138,6 +138,7 @@ def _set_cache_(self, attr: str) -> None:
             stream = file_contents_ro(fd, stream=True, allow_mmap=True)
 
             self._deserialize(stream)
+            os.close(fd)
         else:
             super(IndexFile, self)._set_cache_(attr)