sparse-index: silently return when cache tree fails

If cache_tree_update() returns a non-zero value, then it could not
create the cache tree likely due to a merge conflict. If we remove our
dependence on the cache tree within convert_to_sparse(), then we could
still recover from this scenario and have a sparse index. Since we are
already returning early, let's return silently to avoid making it seem
like we failed to write the index at all.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

sparse-index.c

@@ -179,10 +179,12 @@ int convert_to_sparse(struct index_state *istate)
 
 	/* Clear and recompute the cache-tree */
 	cache_tree_free(&istate->cache_tree);
-	if (cache_tree_update(istate, 0)) {
-		warning(_("unable to update cache-tree, staying full"));
-		return -1;
-	}
+	/*
+	 * Silently return if there is a problem with the cache tree update,
+	 * which might just be due to a conflict state in some entry.
+	 */
+	if (cache_tree_update(istate, 0))
+		return 0;
 
 	remove_fsmonitor(istate);