Refine the libsvn_fs_fs rev file format a little.
Refine the libsvn_fs_fs rev file format a little.

* libsvn_fs_fs/structure: It's not practical to encode the length of
  a text rep at the beginning of the rep itself, because don't know
  how much svndiff data we're going to write until we've written it.
  So put that information in the references to the text reps instead.
  Also, when props and text reps are unmodified, we should just put
  in a pointer to the old representations instead of making new ones.
  So we need a rev in the "rep" and "props" field.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@849369 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion