svnadmin setlog — Set the log message on a revision.
Set the log message on
            revision REVISION to the
            contents of FILE.
This is similar to using svn propset
            with the --revprop option to set the svn:log property
            on a revision, except that you can also use the option
            --bypass-hooks to avoid running any pre-
            or post-commit hooks, which is useful if the modification
            of revision properties has not been enabled in
            the pre-revprop-change hook.
Revision properties are not under version control, so this command will permanently overwrite the previous log message.