... by first adding a preliminary index entry and making this fail if
an index entry for the same blob already exists.
A preliminary index entry is characterized by not yet being associated
with a pack. Until now, these entries where added to the index just
like final index entries using index.Store, which silently overwrites
existing index entries.
This commit adds a new method index.StoreInProgress which refuses to
overwrite existing index entries and allows for creating preliminary
index entries only. The existing method index.Store has not been
changed and continues to silently overwrite existing index entries.
This distinction is important, as otherwise, it would be impossible to
update a preliminary index entry after the blob has been written to a
pack.
Resolves: restic#292
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.
