lib/pull: Update summary checksum code to use new checksum API
lib/pull: Update summary checksum code to use new checksum API

This is another case where making an input stream out of a memory buffer is a
bit silly; just hash the `GBytes` directly.

Closes: #1287
Approved by: jlebon

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