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 <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: Haizzz

sparse-index.c

@@ -510,6 +510,8 @@ void clear_skip_worktree_from_present_files(struct index_state *istate)
 			path_count[restarted]++;
 			if (path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {
 				if (S_ISSPARSEDIR(ce->ce_mode)) {
+					if (restarted)
+						BUG("ensure-full-index did not fully flatten?");
 					ensure_full_index(istate);
 					restarted = 1;
 					goto restart;