Minor cleanups for master

A few things I noticed when I updated recently...

* `FLAG_BITS` seems to be internal only so I moved it to the appropriate internal header. If it needs to be external, it should probably have a `GIT_` prefix or something like that.
* For POSIX functions, we should use `cl_must_pass` and not `cl_git_pass` so a failure won't yield a spurious internal error message.