Release 2021.5

In this release, the sysroot code now ignores (with a log) any non-regular/non-symlink files in `/etc`.   See: https://github.com/ostreedev/ostree/issues/2446

There are a few fixes and API additions for the new "ostree container" work happening in Rust in: https://github.com/ostreedev/ostree-rs-ext/

Some internal preparatory work landed for making an `AutoTransaction` API for C.

Finally, some small test suite improvements landed.

```
Colin Walters (11):
      tests/pull-test: Avoid duplicating test numbers
      tests: Add new TAP APIs
      bin/commit: Fix --tree=tar with --selinux-policy
      tests: Use ostree-ext 0.3.0
      fsck: Print a success message
      repo: Add an API to init `OstreeSePolicy` from commit directly
      sepolicy: Add deprecation comment for `_get_path()`
      lib: Add an API to construct a `MutableTree` from a commit
      deploy: Ignore sockets, fifos in /etc during merge
      Release 2021.5
      configure: post-release version bump

Luca BRUNO (1):
      repo/private: allow committing/aborting through a transaction guard
```

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