On the fsfs-format7 branch:  introduce a very early and inefficent
version of the star-deltifying representation container.  Support
for deltification against representations outside the container is
provided by the API but not the implementation.  Also, there is no
generation of instruction sub-sequences etc.

* subversion/include/svn_string.h
  (svn_stringbuf_appendfill): declare a new API function
* subversion/libsvn_subr/string.c
  (svn_stringbuf_appendfill): implement

* subversion/libsvn_fs_fs/reps.h
  (): new file; provide container API
* subversion/libsvn_fs_fs/reps.c
  (): new file; implement some of the API


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1489949 13f79535-47bb-0310-9956-ffa450edef68