Remove upload-tar and make git-tar-tree a thin wrapper to git-archive

Remove upload-tar and make git-tar-tree a thin wrapper to git-archive

The command now issues a big deprecation warning message and runs
git-archive command with appropriate arguments.

git-tar-tree $tree_ish $base always forces $base to be the leading
directory name, so the --prefix parameter passed internally to
git-archive is a slash appended to it, i.e. "--prefix=$base/".

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