rebase: fix two ui.logs to actually have text when using default blackbox log

Authored by spectral.

Description

rebase: fix two ui.logs to actually have text when using default blackbox log

Some implementations of ui.log record structured information along with the
ui.log which can be used for metrics, but ui.log() as implemented by the
blackbox logging does not do anything special with this, and we end up with a
log line with no text (not even a line break) so it ends up looking something
like:

date time user @node (pid) [rebase]> date time user @node (pid) ...

Differential Revision: https://phab.mercurial-scm.org/D5279