Minor simplification. No functional change.

Found by: Dmitry Pavlenko <pavlenko{_AT_}tmatesoft.com>

* subversion/libsvn_wc/merge.c
  (merge_text_file): Remove a redundant 'if' condition, and simplify the
    associated flow of control a little.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481095 13f79535-47bb-0310-9956-ffa450edef68