import-tars: Allow per-tar author and commit message.

import-tars: Allow per-tar author and commit message.

If the "--metainfo=<ext>" option is given on the command line, a file
called "<filename.tar>.<ext>" will be used to create the commit message
for "<filename.tar>", instead of using "Imported from filename.tar".

The author and committer of the tar ball can also be overridden by
embedding an "Author:" or "Committer:" header in the metainfo file.

Signed-off-by: Peter Krefting <peter@softwolves.pp.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>