Improve DAG node / path lookup performance in FSFS.  Since this is a hot
path in many fs_repo-based functions (like fs_verify), 20 .. 30% time
saved here can easily become significant.

The idea here is that get_dag() already has knowledge about the PATH
parameter and does not actually use all the result data returned by
open_path(). So, define a number of flags that allow a caller to tell
open_path() where it may take short-cuts based on the context.

* subversion/libsvn_fs_fs/tree.c
  (dag_node_cache_set): add comment to document a rationale
  (enum open_path_flags_t): declare further flags
  (open_path): support the new flags to short-circuit portions of code
  (get_dag,
   fs_closest_copy,
   get_mergeinfo_for_path_internal): provide flags to open_path()


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442088 13f79535-47bb-0310-9956-ffa450edef68