lib/repo-pull: Factor out enqueue function for FetchObjectData
lib/repo-pull: Factor out enqueue function for FetchObjectData

This introduces no functional changes, but will make upcoming support
for retrying downloads easier to add.

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

Closes: #1599
Approved by: jlebon

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