Tighten input requirements for a function to simply its documentation
and input validation.

* subversion/include/svn_client.h
  (svn_client_mergeinfo_log2): Update docstring to explain that
    source_start_revision and source_end_revision must have matching
    levels of specificity, and otherwise rework it for clarity.

* subversion/libsvn_client/mergeinfo.c
  (svn_client_mergeinfo_log2): Require source_start_revision and
    source_end_revision to either both be specified, or both be
    unspecified.

* subversion/svn/mergeinfo-cmd.c
  (svn_cl__mergeinfo): Tweak to honor new requirements for
    'source_start_revision' and 'source_end_revision' parameters of
    svn_client_mergeinfo_log2().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467013 13f79535-47bb-0310-9956-ffa450edef68