Merge branch 'jc/diff-no-index-in-subdir' into maint-2.51

"git diff --no-index" run inside a subdirectory under control of a Git repository operated at the top of the working tree and stripped the prefix from the output, and oddballs like "-" (stdin) did not work correctly because of it. Correct the set-up by undoing what the set-up sequence did to cwd and prefix. * jc/diff-no-index-in-subdir: diff: --no-index should ignore the worktree