open_path() is a central function in all API calls to FSFS repos, so it
can always use a bit of tuning.  This time, we replace the O(N^2) usage
of svn_fspath__join() with a linear stringbuf traversal.

* subversion/libsvn_fs_fs/tree.c
  (open_path): copy path to a stringbuf and simply shift the terminator
               position while iterating over the path instead of copying
               it at every iteration.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483301 13f79535-47bb-0310-9956-ffa450edef68