Allocate the merge_stack array for the log receivers on demand.

This ends up saving a tiny amount of memory when log is called without -g and
for mergeinfo --log.

* subversion/svn/log-cmd.c
  (svn_cl__log_entry_receiver, svn_cl__log_entry_receiver_xml):  Check the
    merge_stack member before use and make an array if it hasn't already been
    made before adding a member.
  (svn_cl__log): Initialize the merge_stack member of the baton to NULL.

* subversion/svn/mergeinfo-cmd.c
  (mergeinfo_log): Initialize the merge_stack member of the baton to NULL.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548334 13f79535-47bb-0310-9956-ffa450edef68