repo: Fix object storage size API to be 64 bit

This fixes a pointer size warning on 32-bit builds.

https://bugzilla.gnome.org/show_bug.cgi?id=706235
pull: Drop obsoleted "related objects" API

We removed support for writing "related objects" from ostree commits
in ostree git c9b61cbfee5d3cd0433ef14eac9ba9d1b24a0e38 because it just
didn't work out as an idea.  This also removes the API and code from
"ostree pull".

Note there was no test suite coverage.

https://bugzilla.gnome.org/show_bug.cgi?id=706342
libostree: Improve commit filter API

Make the structure private, and document the flags.

https://bugzilla.gnome.org/show_bug.cgi?id=706214
main: Code cleanup by passing OstreeRepo * directly to builtins

It turns out every builtin (with one special exception) that takes a
repo argument did the same thing; let's just centralize it.  The
special exception was "ostree init --repo=foo" where foo is expected
to *not* actually be a repo.  In that case, simply skip the
ostree_repo_check() invocation.

https://bugzilla.gnome.org/show_bug.cgi?id=706762