sparse-index: fix index.sparse inline usage

Update behavior when the `index.sparse` config setting is provided inline to
a command.  Previously would run command on sparse index (unless otherwise
expanded), then write out as sparse. Now, the index will be expanded in the
process of reading it in.

Co-authored-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Victoria Dye <vdye@github.com>

read-cache.c

@@ -2411,7 +2411,8 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	 * settings and other properties of the index (if necessary).
 	 */
 	prepare_repo_settings(istate->repo);
-	if (istate->repo->settings.command_requires_full_index)
+	if (!istate->repo->settings.sparse_index ||
+	    istate->repo->settings.command_requires_full_index)
 		ensure_full_index(istate);
 	else
 		ensure_correct_sparsity(istate);