Merge branch 'hy/diff-lazy-fetch-with-break-fix'

A prefetch call can be triggered to access a stale diff_queue entry
after diffcore-break breaks a filepair into two and freed the
original entry that is no longer used, leading to a segfault, which
has been corrected.

* hy/diff-lazy-fetch-with-break-fix:
  diffcore-break: avoid segfault with freed entries