Revision 1872104

Raise an error on trying to convert invalid mergeinfo to a string.

Instead of converting invalid mergeinfo to a diagnostic string (r1872030),
immediately raise an error so as not to risk the diagnostic string finding
its way into versioned data.

A follow-up to r1872030.

For issue #4840, "Merge assertion failure in svn_sort__array_insert".

Suggested by: danielsh

* subversion/libsvn_subr/mergeinfo.c
  (range_to_string): Raise an error on invalid input.
  (range_to_string_debug): New.
  (svn_rangelist__canonicalize): Use range_to_string_debug() within an error
    message.
  (svn_rangelist_to_string): Update the calls to range_to_string().