Fix progress output for pkg_add

Reported by andrew clarke:
http://lists.freebsd.org/pipermail/freebsd-ports/2014-August/094883.html

Before:

pkg add tmux-1.8.txz
[pkgtest] Installing libevent-1.4.14b_2: 100%
[pkgtest] Installing libevent-1.4.14b_2: 100%

After:
pkg add tmux-1.8.txz
[pkgtest] Installing libevent-1.4.14b_2: 100%
[pkgtest] Installing tmux-1.8: 100%