restic/changelog/unreleased/pull-3048

Enhancement: Check now has more checks for consistency of index and pack files

Restic used to only verify the pack file content when calling `check --read-data` or
`check --read-data-subset` but did not check if the blobs within the pack are
correctly contained in the index.
This check is now added and may give an "Blob ID is not contained in index or position
is incorrect" error.
Also a new test is added which compares pack file sizes computed from the index and the
pack header with the actual file size. This test is able to detect truncated pack files.

If the index is not correct, it can be rebuilt by using the `rebuild-index` command.

Having added these tests, `restic check` is now able to detect non-existing blobs which
are wrongly referenced in the index. This situation could have lead to missing data.

https://github.com/restic/restic/pull/3048
https://github.com/restic/restic/pull/3082