`magit-log': remove optional EXTRA-ARGS arg

Added in commit 0a9bf300, was used for passing extra options to
git-log(1), but we no longer have any callers that pass more than
one arg to `magit-log'.  This also allows us to replace the
`append' + `list' with `cons', as is used in `magit-log-long'.

Signed-off-by: Pieter Praet <pieter@praet.org>