OSTree v2015.5

[Colin Walters <walters verbum org>]

I'm trying to do more frequent releases now to match the desire
to do faster releases of Project Atomic.

Most notable in this release is that it fixes a bug that could be hit
on some notable web servers (e.g. Akamai) that broke OSTree's
internal HTTP caching.  See the commit message in
https://github.com/GNOME/ostree/pull/84
for more details.

Otherwise, mostly smaller fixes, along with a welcome merge of
work from Matthew to show a nice "you must be root" error if
you happen to try to do root-owned ostree bits as non-root.

$ git shortlog v2015.4..v2015.5
Colin Walters (6):
      main: Only verify SUPERUSER flag if using default sysroot
      dist-packaging: Don't delete 91-ostree.preset, do clean old rpms/sources
      tests: Verify that the pull error was from interruption
      pull: Handle remote web server not honoring range requests
      ostree_repo_checkout_tree_at: New API for checkouts
      Release 2015.5

Daniel Svensson (2):
      build: Use glibc's xattr support instead of requiring libattr
      tests: Missing linker flags for test-rollsum

Giuseppe Scrivano (6):
      bsdiff: change submodule location
      build: Drop libattr from the spec file
      tests/basic-test.sh: enable repo-noperm test only for non-root user
      src/ostree/ot-main.c: drop empty newline at end of file
      build: exclude .sig files from syntax-check
      tests: skip test-commit-sign.sh when not root

Matthew Barnes (8):
      Add ostree_repo_is_writable()
      Add ostree_ensure_repo_writable()
      Check repo permission prior to attempting to modify it
      tests: Add a test case for unwritable repos
      Add OstreeAdminBuiltinFlags for admin commands
      Add OSTREE_ADMIN_BUILTIN_FLAG_SUPERUSER
      Include ostree-gpg-verify-result.h in ostree.h
      core: Actually allow none in ostree_parse_refspec()

Sam Thursfield (1):
      core: Fix possible crash in ostree_mutable_tree_walk()