Merge branch 'jk/diff-do-not-reuse-wtf-needs-cleaning' into maint

There is an optimization used in "git diff $treeA $treeB" to borrow an already checked-out copy in the working tree when it is known to be the same as the blob being compared, expecting that open/mmap of such a file is faster than reading it from the object store, which involves inflating and applying delta. This however kicked in even when the checked-out copy needs to go through the convert-to-git conversion (including the clean filter), which defeats the whole point of the optimization. The optimization has been disabled when the conversion is necessary. * jk/diff-do-not-reuse-wtf-needs-cleaning: diff: do not reuse worktree files that need "clean" conversion