Move git_reference__alloc to include/git2/sys
Create a new include/git2/sys/refs.h and move the reference alloc
functions there. Also fix some documentation issues and some
minor code cleanups.
Move refdb_backend to include/git2/sys
This moves most of the refdb stuff over to the include/git2/sys
directory, with some minor shifts in function organization.
While I was making the necessary updates, I also removed the
trailing whitespace in a few files that I modified just because I
was there and it was bugging me.