make oneline reflog dates more consistent with multiline format

make oneline reflog dates more consistent with multiline format

The multiline reflog format (e.g., as shown by "git log -g")
will show HEAD@{<date>} rather than HEAD@{<count>} in two
situations:

  1. If the user gave branch@{<date>} syntax to specify the
     reflog

  2. If the user gave a --date=<format> specifier

It uses the "normal" date format in case 1, and the
user-specified format in case 2.

The oneline reflog format (e.g., "git reflog show" or "git
log -g --oneline") will show the date in the same two
circumstances. However, it _always_ shows the date as a
relative date, and it always ignores the timezone.

In case 2, it seems ridiculous to trigger the date but use a
format totally different from what the user requested.

For case 1, it is arguable that the user might want to see
the relative date by default; however, the multiline version
shows the normal format.

This patch does three things:

  - refactors the "relative_date" parameter to
    show_reflog_message to be an actual date_mode enum,
    since this is how it is used (it is passed to show_date)

  - uses the passed date_mode parameter in the oneline
    format (making it consistent with the multiline format)

  - does not ignore the timezone parameter in oneline mode

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>