sparse-index: log failure to clear skip-worktree

The clear_skip_worktree_from_present_files_sparse() method attempts to
clear the skip worktree bit from cache entries in the index depending on
when they exist in the workdir. When this comes across a sparse
directory that actually exists in the workdir, then this method fails
and signals that the index needs expansion.

The index expansion already logs a reason, but this reason is separate
from the path that caused this failure.

Add logging to demonstrate this situation for full clarity.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

sparse-index.c

@@ -635,6 +635,8 @@ static int clear_skip_worktree_from_present_files_sparse(struct index_state *ist
 			if (path_found(ce->name, &data)) {
 				if (S_ISSPARSEDIR(ce->ce_mode)) {
 					to_restart = 1;
+					trace2_data_string("sparse-index", istate->repo,
+							   "skip-worktree sparsedir", ce->name);
 					break;
 				}
 				ce->ce_flags &= ~CE_SKIP_WORKTREE;