git-tar-tree: devolve git-tar-tree into a wrapper for git-archive

git-tar-tree: devolve git-tar-tree into a wrapper for git-archive

This patch removes the custom tree walker tree_traverse(), and makes
generate_tar() use write_tar_archive() and the infrastructure provided
by git-archive instead.

As a kind of side effect, make write_tar_archive() able to handle NULL
as base directory, as this is what the new and simple generate_tar()
uses to indicate the absence of a base directory.  This was simpler
and cleaner than playing tricks with empty strings.

The behaviour of git-tar-tree should be unchanged (quick tests didn't
indicate otherwise) except for the text of some error messages.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>