repack-promisor: preserve content of promisor files after repack

repack-promisor: preserve content of promisor files after repack

When a `repack` involving promisor packfiles happens, the new ".promisor"
file is created empty, losing all the debug info that might be present
inside the ".promisor" files before the `repack`.

Use the previously created "write_promisor_file_after_repack()" function
to preserve the contents of all ".promisor" files inside the ".promisor"
files created by the `repack`.

For geometric repacking, we have to create a `strset` that contains the
basenames of all excluded packs. For "normal" repacking this is not
necessary, since there should be no excluded packs.

Also, update the documentation accordingly.

Helped-by: Tian Yuchen <cat@malon.dev>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: LorenzoPegorari <lorenzo.pegorari2002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>