load_branch_decorations: fix memory leak with non-static filters

load_branch_decorations: fix memory leak with non-static filters

load_branch_decorations calls normalize_glob_ref on each string of filter's
string_lists. This effectively replaces the potentially non-owning char* of
those items with an owning char*.

Set the strdup_string flag on those string_lists.

This was not caught until now because:
- when passing string_lists already with the strdup_string already set, the
  behaviour was correct
- when passing static string_lists, the new char* remain reachable until
  program exit

Signed-off-by: Nicolas Guichard <nicolas@guichard.eu>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>