In membuffer caching, simplify the L1->L2 promotion logic.  No longer
prevent cache index entries pertaining to L1 from being moved to another
entry in the same group.  Check for that special condition in the
caller now where it is more obvious.

* subversion/libsvn_subr/cache-membuffer.c
  (promote_entry): assert that we actually call this for L1 entries only
  (ensure_data_insertable_l2): don't care whether we move L1 entries
  (ensure_data_insertable_l1): check for moved L1 entries here

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519272 13f79535-47bb-0310-9956-ffa450edef68