function diff_cleanupMerge(diffs) {
  diffs.push([DIFF_EQUAL, '']);  // Add a dummy entry at the end.
  var pointer = 0;
  var count_delete = 0;
  var count_insert = 0;
  var text_delete = '';
  var text_insert = '';
  var commonlength;
  while (pointer < diffs.length) {
    switch (diffs[pointer][0]) {
      case DIFF_INSERT:
        count_insert++;
        text_insert += diffs[pointer][1];
        pointer++;
        break;
      case DIFF_DELETE:
        count_delete++;
        text_delete += diffs[pointer][1];
        pointer++;
        break;
      case DIFF_EQUAL:
        // Upon reaching an equality, check for prior redundancies.
        if (count_delete + count_insert > 1) {
          if (count_delete !== 0 && count_insert !== 0) {
            // Factor out any common prefixies.
            commonlength = diff_commonPrefix(text_insert, text_delete);
            if (commonlength !== 0) {
              if ((pointer - count_delete - count_insert) > 0 &&
                  diffs[pointer - count_delete - count_insert - 1][0] ==
                  DIFF_EQUAL) {
                diffs[pointer - count_delete - count_insert - 1][1] +=
                    text_insert.substring(0, commonlength);
              } else {
                diffs.splice(0, 0, [DIFF_EQUAL,
                                    text_insert.substring(0, commonlength)]);
                pointer++;
              }
              text_insert = text_insert.substring(commonlength);
              text_delete = text_delete.substring(commonlength);
            }
            // Factor out any common suffixies.
            commonlength = diff_commonSuffix(text_insert, text_delete);
            if (commonlength !== 0) {
              diffs[pointer][1] = text_insert.substring(text_insert.length -
                  commonlength) + diffs[pointer][1];
              text_insert = text_insert.substring(0, text_insert.length -
                  commonlength);
              text_delete = text_delete.substring(0, text_delete.length -
                  commonlength);
            }
          }
          // Delete the offending records and add the merged ones.
          if (count_delete === 0) {
            diffs.splice(pointer - count_insert,
                count_delete + count_insert, [DIFF_INSERT, text_insert]);
          } else if (count_insert === 0) {
            diffs.splice(pointer - count_delete,
                count_delete + count_insert, [DIFF_DELETE, text_delete]);
          } else {
            diffs.splice(pointer - count_delete - count_insert,
                count_delete + count_insert, [DIFF_DELETE, text_delete],
                [DIFF_INSERT, text_insert]);
          }
          pointer = pointer - count_delete - count_insert +
                    (count_delete ? 1 : 0) + (count_insert ? 1 : 0) + 1;
        } else if (pointer !== 0 && diffs[pointer - 1][0] == DIFF_EQUAL) {
          // Merge this equality with the previous one.
          diffs[pointer - 1][1] += diffs[pointer][1];
          diffs.splice(pointer, 1);
        } else {
          pointer++;
        }
        count_insert = 0;
        count_delete = 0;
        text_delete = '';
        text_insert = '';
        break;
    }
  }
  if (diffs[diffs.length - 1][1] === '') {
    diffs.pop();  // Remove the dummy entry at the end.
  }

  // Second pass: look for single edits surrounded on both sides by equalities
  // which can be shifted sideways to eliminate an equality.
  // e.g: A<ins>BA</ins>C -> <ins>AB</ins>AC
  var changes = false;
  pointer = 1;
  // Intentionally ignore the first and last element (don't need checking).
  while (pointer < diffs.length - 1) {
    if (diffs[pointer - 1][0] == DIFF_EQUAL &&
        diffs[pointer + 1][0] == DIFF_EQUAL) {
      // This is a single edit surrounded by equalities.
      if (diffs[pointer][1].substring(diffs[pointer][1].length -
          diffs[pointer - 1][1].length) == diffs[pointer - 1][1]) {
        // Shift the edit over the previous equality.
        diffs[pointer][1] = diffs[pointer - 1][1] +
            diffs[pointer][1].substring(0, diffs[pointer][1].length -
                                        diffs[pointer - 1][1].length);
        diffs[pointer + 1][1] = diffs[pointer - 1][1] + diffs[pointer + 1][1];
        diffs.splice(pointer - 1, 1);
        changes = true;
      } else if (diffs[pointer][1].substring(0, diffs[pointer + 1][1].length) ==
          diffs[pointer + 1][1]) {
        // Shift the edit over the next equality.
        diffs[pointer - 1][1] += diffs[pointer + 1][1];
        diffs[pointer][1] =
            diffs[pointer][1].substring(diffs[pointer + 1][1].length) +
            diffs[pointer + 1][1];
        diffs.splice(pointer + 1, 1);
        changes = true;
      }
    }
    pointer++;
  }
  // If shifts were made, the diff needs reordering and another shift sweep.
  if (changes) {
    diff_cleanupMerge(diffs);
  }
}
Reorder and merge like edit sections.  Merge equalities.
Any edit section can move as long as it doesn't cross an equality.
@param {Array} diffs Array of diff tuples.