lib/repo: Restore tmpdir reusing out parameter
lib/repo: Restore tmpdir reusing out parameter

This got lost in d0b0578 and now the caller always thinks it got a new
tmpdir.

Closes: #1346
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree