completion: complete some 'git log' options

completion: complete some 'git log' options

1. '--exclude=' option to 'git log' and 'git shortlog' are missing. Add the
option to __git_log_shortlog_options.

2. The `--committer` option in `git log` requires a pattern, such as
`--committer=ba`, but in `git shortlog`, specifying a pattern results in
an error: “error: option `committer' takes no value.” Handle them as
separate options for completion rather than a shared one.

Signed-off-by: KIYOTA Fumiya <aimluck.kiyota@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>