Date for comment with related activity is date of comment, not of earliest event in the group

[Gavin Panella]

When a comment is grouped with activity on a bug page, the date given
for the group is the date of the comment, even if the comment occurred
after the activity with which it is grouped.

To reduce confusion, either the earliest date of the grouped events
should be shown, or a date *range*, for example:

  Gavin Panella, between 7 and 3 minutes ago, ...