Add refdb.h to git2.h, reorder git2.h sanely

`refdb.h` should be included in `git2.h`.  When adding said file, I realized that `git2.h` is sort of confusing.  At first, I wasn't sure where to put that line.  I thought the file might be laid out by functional area, so I puzzled over that for a moment but that did not appear to be the case.  Then I thought the file might be laid out by higher-level and lower-level functions, so I puzzled over that for a moment but that did not appear to be the case either.  So I said fuck it and ordered it alphabetically.