lib/repo: Add min-free-space-percent option, default 3%

For ostree-as-host, we're the superuser, so we'll blow past
any reserved free space by default.  While deltas have size
metadata, if one happens to do a loose fetch, we can fill
up the disk.

Another case is flatpak: the system helper has similar concerns
here as ostree-as-host, and for `flatpak --user`, we also
want to be nice and avoid filling up the user's quota.

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

Closes: #987
Approved by: jlebon

tests/installed/itest-pull-space.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Test min-free-space-percent using loopback devices
+
+set -xeuo pipefail
+
+dn=$(dirname $0)
+. ${dn}/libinsttest.sh
+
+test_tmpdir=$(prepare_tmpdir)
+trap _tmpdir_cleanup EXIT
+
+cd ${test_tmpdir}
+truncate -s 100MB testblk.img
+blkdev=$(losetup --find --show $(pwd)/testblk.img)
+mkfs.xfs ${blkdev}
+mkdir mnt
+mount ${blkdev} mnt
+ostree --repo=mnt/repo init --mode=bare-user
+if ostree --repo=mnt/repo pull-local /ostree/repo ${host_commit} 2>err.txt; then
+    fatal "succeeded in doing a pull with no free space"
+fi
+assert_file_has_content err.txt "min-free-space-percent"
+umount mnt
+losetup -d ${blkdev}