Fix an interactive conflict resolution buglet (just displaying an extra
option in one case) and a coding style issue.  A follow-up to r1416578.

* subversion/svn/conflict-callbacks.c
  (ARRAY_LEN): New macro.
  (handle_text_conflict): Use it instead of a
    magic number. Omit not only 'mc' but also 'tc' for a binary file.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1434917 13f79535-47bb-0310-9956-ffa450edef68