reflog: show committer date in verbose mode

By default, reflog won't show committer date and for some cases won't
show commit log either. It will be helpful to show them all by passing
a more complicated pretty formatter to `git reflog` like this:

    $ git reflog show \
      --pretty="%Cred%h%Creset %gd: %gs%n  >> %Cblue%ci (%cr)%Creset: %s"

It will be nice to add this pretty formatter automatically when run
`git reflog` in verbose mode. And in order to support verbose mode, add
new flag "verbose" in struct rev_info.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>