Apply some minor comment and code tweaks based on reviews of todays changes.

* subversion/libsvn_client/merge.c
  (merge_cmd_baton_t): Tweak comment.
  (remove_first_range_from_remaining_ranges): Remove outdated comment.
    A simple memmove() doesn't allocate. And copying a list of mergeranges
    is not a performance problem.
  (do_file_merge): Use better array remove. Add comment to point to the
    directory equivalent.
  (do_mergeinfo_unaware_dir_merge): Tweak comment.
  (do_mergeinfo_aware_dir_merge): Tweak comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442787 13f79535-47bb-0310-9956-ffa450edef68