Eliminate another 50% of the revprop file header parsing overhead.

Unless we want to modify the pack file, i.e. write a revprop, we are not
interested in anything but a single revision's revprops.  So, add an
READ_ALL flag to the parser and let the callers decide what they need.

* subversion/libsvn_fs_fs/revprops.c
  (parse_packed_revprops): Read the revprop location and size info only
                           if the READ_ALL flag is set. 
  (read_pack_revprop): Add READ_ALL parameter as a pass-through.
  (svn_fs_fs__get_revision_proplist): We only need one set of revprops
                                      and won't use the pack file contents
                                      for anything else.
  (write_packed_revprop): We need the full packed revprop info here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1634879 13f79535-47bb-0310-9956-ffa450edef68