git, help: fix memory leaks in alias listing

The list_aliases() function sets the util pointer of each list item to
a heap-allocated copy of the alias command value.  Two callers failed
to free these util pointers:

 - list_cmds() in git.c collects a string list with STRING_LIST_INIT_DUP
   and clears it with string_list_clear(&list, 0), which frees the
   duplicated strings (strdup_strings=1) but not the util pointers.
   Pass free_util=1 to free them.

 - list_cmds_by_config() in help.c calls string_list_sort_u(list, 0) to
   deduplicate the list before processing completion.commands overrides.
   When duplicate entries are removed, the util pointer of each discarded
   item is leaked because free_util=0.  Pass free_util=1 to free them.

Reported-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -119,7 +119,7 @@ static int list_cmds(const char *spec)
 	}
 	for (size_t i = 0; i < list.nr; i++)
 		puts(list.items[i].string);
-	string_list_clear(&list, 0);
+	string_list_clear(&list, 1);
 	return 0;
 }