Fix bug where incorrect use of 'svn diff' was dying silently.
Fix bug where incorrect use of 'svn diff' was dying silently.

* main.c (main): why on earth does this function just exit(1) silently
  whenever it might potentially handle a SVN_ERR_CL_ARG_PARSING_ERROR?
  Why is that particular error ignored, but all others printed?
  Whatever the case, at least print help on the subcommand.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion