Fewer uses of size_t in incremen.c

Fewer uses of size_t in incremen.c

* src/incremen.c (struct dumpdir, dumpdir_create0, struct dumpdir_iter)
(dumpdir_next, dumpdir_size, make_directory)
(dirlist_replace_prefix, rebase_directory, makedumpdir)
(maketagdumpdir, append_incremental_renames, read_obstack)
(read_incr_db_2, get_gnu_dumpdir, try_purge_directory)
(list_dumpdir): Prefer idx_t to size_t.