lfs: avoid quadratic performance in processing server responses
This is also adapted from the Facebook repo[1]. Unlike there, we were already
reading the download stream in chunks and immediately writing it to disk, so we
basically avoided the problem on download. There shouldn't be a lot of data to
read on upload, but it's better to get rid of this pattern.
[1] https://github.com/facebookexperimental/eden/commit/
82df66ffe97e21f3ee73dfec093c87500fc1f6a7
Differential Revision: https://phab.mercurial-scm.org/D7882
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.