daemon: add upload-tar service.

daemon: add upload-tar service.

This allows clients to ask for tarballs with:

	git tar-tree --remote=git://server/repo refname

By default, the upload-tar service is not enabled.  To enable
it server-wide, the server can be started with:

	git-daemon --enable=upload-tar

This service is by default overridable per repostiory, so
alternatively, a repository can define "daemon.uploadtar = true"
to enable it.

Signed-off-by: Junio C Hamano <junkio@cox.net>