This invocation of run_status() is asked to find out if there is a
committable change, but it unconditionally gave instructions such as
"use git-add" at the same time. When in merge, we do allow an empty
change to be recorded, so after showing this message the code still went
ahead and made a commit.
This introduces "nowarn" parameter to run_status() to avoid these
useless messages. If we are not allowed to create an empty commit, we
already call run_status() again in the original codepath, and the
message will be shown from that call anyway.