Make the get_default_head() function more generic.

* contrib/client-side/svnmerge.py
  (get_default_head): Removed.
  (get_heads): New function similar to get_default_head(), but can return
  all the available head values. Also takes an extra parameter
  `get_only_default'. If `get_only_default' is True, expect only one
  head (error out if not), and return it. If `get_only_default' is
  False, return all the available heads as a list.
  (main): Modify call of get_default_head() to get_heads().