reflog: change one->many worktree->refnames to use a string_list

reflog: change one->many worktree->refnames to use a string_list

Change the FLEX_ARRAY pattern added in bda3a31cc79 (reflog-expire:
Avoid creating new files in a directory inside readdir(3) loop,
2008-01-25) the string-list API instead.

This does not change any behavior, allows us to delete much of this
code as it's replaced by things we get from the string-list API for
free, as a result we need just one struct to keep track of this data,
instead of two.

The "DUP" -> "string_list_append_nodup(..., strbuf_detach(...))"
pattern here is the same as that used in a recent memory leak fix in
b202e51b154 (grep: fix a "path_list" memory leak, 2021-10-22).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>