import-tars: separate author from committer

import-tars: separate author from committer

The import-tars script is typically employed to (re)create the past
history of a project from stored tars. Although assigning authorship in
these cases can be a somewhat arbitrary process, it makes sense to set
the author to whoever created the tars in the first place (if it's
known), and (s)he can in general be different from the committer
(whoever is running the script).

Implement this by having separate author and committer data, making them
settable from the usual GIT_* environment variables.

Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>