These commands filter the snapshots according to some criteria which
essentially requires loading the index before filtering the snapshots.
Thus create a copy of the snapshots list beforehand and use it later on.
This enables the backends to request the calculation of a
backend-specific hash. For the currently supported backends this will
always be MD5. The hash calculation happens as early as possible, for
pack files this is during assembly of the pack file. That way the hash
would even capture corruptions of the temporary pack file on disk.
This assigns an id to each tree root and then keeps track of how many
tree loads (i.e. trees referenced for the first time) are pending per
tree root. Once a tree root and its subtrees were fully processed there
are no more pending tree loads and the tree root is reported as
processed.
UnusedBlobs now directly reads the list of existing blobs from the
repository index. This removes the need for the blobStatusExists flag,
which in turn allows converting the blobRefs map into a BlobSet.
By construction these two errors always show up in pairs: 'size could
not be found' is printed when the blob is not found in the repository
index. That blob is also part of the `blobs` array. Later on, check
iterates over that array and checks whether the blob is marked as
existing. Which cannot be the case as that mark is generated by
iterating over the repository index.
The merged warning no longer reports the blob index within a file. That
information could also be derived by printing the affected tree using
`cat` and searching for the blob.
If a data blob and a tree blob with the same ID (= same content) exist,
then the checker did not report a data or tree blob as unused when the
blob of the other type was still in use.
The `DuplicateTree` flag is necessary to ensure that failures cannot be
swallowed. The old checker implementation ignores errors from LoadTree
if the corresponding tree was already checked.
Backups traverse the file tree in depth-first order and saves trees on
the way back up. This results in tree packs filled in a way comparable
to the reverse Polish notation. In order to check tree blobs in that
order, the treeFilter would have to delay the forwarding of tree nodes
until all children of it are processed which would complicate the
implementation.
Therefore do the next similar thing and traverse the tree in depth-first
order, but process trees already on the way down. The tree blob ids are
added in reverse order to the backlog, which is once again reverted when
removing the ids from the back of the backlog.
