lfs: explicitly close the file handle for the blob being uploaded The previous code relied on reading the blob fully to close it. The obvious problem is if an error occurs before that point. But there is also a problem when using workers where the data may need to be re-read, which can't happen once it is closed. This eliminates the surprising behavior before attempting to fix the worker problem. Differential Revision: https://phab.mercurial-scm.org/D7957