It was observed that a restic repository still contained overlapping
indexes after `rebuild-index` has been called. This is caused by
instantly forgetting that blobs have already been saved once a full
index has been written during index rebuilding.
This commit adds a (failing) test that shows the behaviour.
Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).
This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.
This adds the exclude patterns used to create a backup in the snapshot,
so we can later compute statistics (like git does) on the data
structure, e.g. added/removed files etc. For that, we need the exclude
pattern.