remotefilelog: add newlines to ui.log() invocations
The comment at the top of ui.log() says that the message should be a
newline-terminated string. When using the blackbox logger, if the string does
not end in a newline, the appearance in the file gets quite messy.
This sometimes leaves a string with just the newline as the message, these are
logged by the blackbox logger just fine. I don't know what other loggers do when
logging structured data and a message that is just the newline.
Differential Revision: https://phab.mercurial-scm.org/D5533