Factor out some repeated code.

* subversion/include/private/svn_fs_private.h,
  subversion/libsvn_fs/fs-loader.c
  (svn_fs__get_mergeinfo_for_path): New, semi-public and modified version of
    a function that was in rev_hunt.c.

* subversion/libsvn_repos/log.c
  (fs_mergeinfo_changed,
   get_combined_mergeinfo_changes): Use it to simplify code.

* subversion/libsvn_repos/rev_hunt.c
  (get_path_mergeinfo): Delete this version of the repeated code.
  (get_merged_mergeinfo): Adjust to use the new version.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570294 13f79535-47bb-0310-9956-ffa450edef68