index: raise a bug if the index is materialised more than once

index: raise a bug if the index is materialised more than once

If clear_skip_worktree_from_present_files() encounter a sparse directory,
it fully materialise the index which should expand any sparse directories
and start going through each entries again. If this happens more than once,
raise it with a BUG.

Signed-off-by: Anh Le <redacted>
Signed-off-by: Taylor Blau <redacted>