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