Make `git l` date format overridable

Recommend --word-diff-regex=. to view diff.

%ad and %cd display date with whatever the last date format option is,
so this defaults to relative but you can override it by just doing

    git l --date=rfc

or whatever.
Add `git lt`, "git l with timestamps"

Supplying a date format option to `git l` overrides the date format;
`git lt` includes both the relative date and adds an absolute date
(overridable format, of course).