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

t/t0021-conversion.sh

@@ -268,4 +268,15 @@ test_expect_success 'disable filter with empty override' '
 	test_must_be_empty err
 '
 
+test_expect_success 'diff does not reuse worktree files that need cleaning' '
+	test_config filter.counter.clean "echo . >>count; sed s/^/clean:/" &&
+	echo "file filter=counter" >.gitattributes &&
+	test_commit one file &&
+	test_commit two file &&
+
+	>count &&
+	git diff-tree -p HEAD &&
+	test_line_count = 0 count
+'
+
 test_done