pull: Check free space when pulling deltas
pull: Check free space when pulling deltas

Computing download/storage size for `archive` pulls is hard; there's
`OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES` which was from a
pre-static-deltas effort by Endless, but we aren't currently making use of this
much.

Static deltas were designed to solve this problem; we have the total
uncompressed size. Let's check free space before doing a delta pull.

Related: https://github.com/ostreedev/ostree/issues/962

Closes: #963
Approved by: jlebon

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