Before the 2018.2 release of libostree there was no way to specify
commit IDs when using find_remotes_async(). The latest commit is always
pulled, so flatpak apps can't be downgraded when collection IDs are in
use. Now that an option is provided by libostree this commit uses it,
and updates the minimum required version to 2018.2 when P2P support is
enabled.
The effect is that `flatpak update --commit=HASH APP` will work when APP
comes from a repository that has a collection ID configured.
Fixes https://github.com/flatpak/flatpak/issues/1309Closes: #1314
Approved by: pwithnall