Release 2017.1

This release has mostly bugfixes, the main new feature is that the `prune`
command gained more sophistication around selectively pruning branches. We're
planning to use this in Project Atomic work where we want to co-locate both
"development" and "stable" branches in the same repository.

The next release is likely to be more exciting, as we have an additional new
libcurl backend in the works - this release contains some preparatory cleanup
for that.

Thanks to all contributors!

```
Colin Walters (19):
      docs: Fix ostree.version -> version
      fetcher: Hoist core "mirrored request" API to public
      fetcher: Move high level functions into "fetcher-util"
      fetcher: Split lowlevel API into file/membuf variants
      build-sys: Minor makefile tweaks
      Split trivial-httpd into separate binary
      pull: Rework delta superblock fetches to be async
      trivial-httpd: Daemonize better
      .dir-locals.el: Standard Emacs indentation config
      unlock: Fix description for --hotfix
      tests: Alias assert_not_reached() -> fatal()
      pull: Fix theoretical checksum collision for metadata fetches
      fetcher: Rework API to use strings for tls keys/db
      tests: Don't inject newline in URL
      tests: Loosen error regexp
      libtest: Enable web server logs
      Add support for more selective pruning
      tests: Add a big (many objects) pull
      Release 2017.1

Dan Nicholson (3):
      repo: Fix indentation
      repo: Fix object list keys ownership
      repo: Fix list_objects annotations

Mario Sanchez Prada (1):
      static-delta: Pretend that world unreadable objects are new objects

Paul van Tilburg (1):
      admin: Use execlp() to look for systemctl as the shell would

Simon McVittie (3):
      Fix TAP syntax in test-basic-user.sh, and run it
      Sourced test snippets: remove shebang and make non-executable
      Make corrupt-repo-ref.js executable
```

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