Fix behavior of complete conflicts

we should guard against cases where there are 0 matchings when generating an LCS.

Author: implausible

diff3.js

@@ -58,12 +58,18 @@ function longestCommonSubsequence(file1, file2) {
       }
   }
 
-  prev.chain = {
+  var tail = {
     file1index: -1,
     file2index: -1,
     chain: null
   };
 
+  if (!prev) {
+    return tail;
+  }
+
+  prev.chain = tail;
+
   return root;
 }