lib/pull: Fix construction of a refspec to use the correct separator
lib/pull: Fix construction of a refspec to use the correct separator

This code looks like it was supposed to build a refspec, but it used a
slash as a separator rather than a colon. The following code does
recover by supporting prefix matching with slashes, but it seems like
this was perhaps not the intention.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #912
Approved by: cgwalters

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