ls-files: fix a --with-tree memory leak

Fix a memory leak in overlay_tree_on_index(), we need to
clear_pathspec() at some point, which might as well be after the last
time we use it in the function.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/ls-files.c

@@ -613,6 +613,7 @@ void overlay_tree_on_index(struct index_state *istate,
 	if (!fn)
 		fn = read_one_entry_quick;
 	err = read_tree(the_repository, tree, &pathspec, fn, istate);
+	clear_pathspec(&pathspec);
 	if (err)
 		die("unable to read tree entries %s", tree_name);