Fix content being missed in the build logs

Summary: This fixes an issue where content would be discarded when the content to append is larger than the chunk size limit.

Test Plan: Tested running a remote command that does I=0; while true; do echo "$I"; I=$[$I+1]; done and all of the outputted numbers matched the line numbers in the logs.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7537