archive: implement configurable tar filters
It's common to pipe the tar output produce by "git archive"
through gzip or some other compressor. Locally, this can
easily be done by using a shell pipe. When requesting a
remote archive, though, it cannot be done through the
upload-archive interface.
This patch allows configurable tar filters, so that one
could define a "tar.gz" format that automatically pipes tar
output through gzip.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>