magit-log-oneline-re: add missing statuses

The regex was missing the values for the cases:

 - "U" for a good, untrusted signature, and;

 - "N" for no signature.

Without this, the log buffer in short mode and with
`magit-log-show-gpg-status` evaluating to true, would unconditionally
show the GPG signature status, author and relative date in the message
line as output by git.