builtin/repack: fix leaks when computing packs to repack

builtin/repack: fix leaks when computing packs to repack

When writing an MIDX in git-repack(1) we first collect all the pack
names that we want to add to it in a string list. This list is marked as
`NODUP`, which indicates that it will neither duplicate nor own strings
added to it. In `write_midx_included_packs()` we then `insert()` strings
via `xstrdup()` or `strbuf_detach()`, but the resulting strings will not
be owned by anything and thus leak.

Fix this issue by marking the list as `DUP` and using a local buffer to
compute the pack names.

This leak is hit in t5319, but plugging it is not sufficient to make the
whole test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>