Enhancement: Speed up the `rebuild-index` command

We've optimized the `rebuild-index` command. Now, existing index entries are used
to minimize the number of pack files that must be read. This speeds up the index
rebuild a lot.

Additionally, the option `--read-all-packs` has been added, implementing the
previous behavior.

https://github.com/restic/restic/issue/2547
https://github.com/restic/restic/pull/3006
