Backports #2741, Fixes#1041e93c445 added a feature called early command, which uses `--` as a
prefix to denote some commands that runs ahead of session loading.
While the feature might be useful especially for logging, `--` is too
useful just for this purpose.
In addition, this adds log level commands with single `-`, such as
-error to treat them as early commands; and keeps `--` variant for log
level for backward compatibility.
Eugene Yokota