2019.4

This is mostly a bugfix release. Notably, the 2019.3 release caused some issues
related to the gpg-agent code spewing messages on the terminal. Additionally,
Fedora 31 users have hit upon issues with `ostree-finalize-staged.service`
running too late to be able to write back its logs to the journal. This then
confused `rpm-ostree` after reboot, because it looks at the previous boot's
journal for this message.

The biggest feature-ish change is support for a partial commit "reason" so that
after `ostree fsck --delete` was used, subsequent `ostree fsck` will continue to
report an error.  This should be used by higher level tools that want to do
"fsck and repair".  It's likely at some point that "fsck and repair" logic will
move down into the libostree core as well.

There are ongoing efforts to port Fedora CoreOS to s390x: one fix landed here to
add the deployment prefix to BLS entries since it's what the `zipl` bootloader
expected.

Special thanks to first-time contributors Benjamin Gilbert and Jason Wessel!

---

```
Benjamin Gilbert (1 PR, 1 commit)
  prepare-root: remember to remove /sysroot.tmp (#1919)

Colin Walters (4 PRs, 4 commits)
  Post-release version bump (#1902)
  sysroot: Add a clearer error if /boot/loader isn't found (#1905)
  ci: Add prow/ subdirectory with Dockerfile (#1906)
  fsck: Fix version in docs, tweak error text (#1918)

Dan Nicholson (3 PRs, 3 commits)
  lib/gpg: Only show gpg-connect-agent stderr on failures (#1908)
  lib/gpg: Don't kill gpg-agent on newer gnupg (#1915)
  lib/gpg: Use g_spawn_sync to kill gpg-agent (#1917)

Jason Wessel (1 PR, 2 commits)
  PR: #1910
    fsck: Add test for --delete corruption, fix repair, and partial commit checks
    fsck: Implement a partial commit reason bitmask

Javier Martinez Canillas (2 PRs, 3 commits)
  lib/bootconfig-parser: Always include deployment index in BLS title (#1911)
  PR: #1904
    lib/bootconfig-parser: Write BLS fragment fields in a deterministic order
    lib/bootconfig-parser: Remove support to preserve comments in BLS files

Jonathan Lebon (2 PRs, 3 commits)
  boot/finalize-staged: Run after systemd-journal-flush.service (#1926)
  Release 2019.4 (#1927)

Philip Withnall (1 PR, 1 commit)
  lib/repo-pull: Add more debugging on pull failure (#1925)
```

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