replace magit-log-{verbose,nongraph}-{args => re}

Replace `magit-log-verbose-args' with `magit-log-verbose-re' and
replace `magit-log-nongraph-args' with `magit-log-nongraph-re'.
magit-log-wash-{line,verbose}: fix log-margin's pseudo-fringe
magit-log-show-more-entries: fill doc-string