branchmap: handle nullrev in setcachedata 906be86990 recently changed to switch from: self._rbcrevs[rbcrevidx:rbcrevidx + _rbcrecsize] = rec to pack_into(_rbcrecfmt, self._rbcrevs, rbcrevidx, node, branchidx) This causes an exception if rbcrevidx is -1 (i.e. the nullrev). The old code handled this because python handles out of bound sets to arrays gracefully. The new code throws because the self._rbcrevs buffer isn't long enough to write 8 bytes to. Normally it would've been resized by the immediately preceding line, but because the 0 length buffer is greater than the idx (-1) times the size, no resize happens. Setting the branch for the nullrev doesn't make sense anyway, so let's skip it. This was caught by external tests in the Facebook extensions repo, but I've added a test here that catches the issue.