Enhancement: Speed up rebuild-index

We've optimized the command rebuild-index. 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.
Also the option --read-all-packs was added which implements the previous behavior.

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