…rt 2

Added code for selecting multiple snapshots.
Added message how many packfiles and their cumulative size were selected.
In internal/checker/checker.go replaced the datablob / packfile selection from using walker.Walk
to restic.StreamTrees - parallelizing the packfile selection.

resolved conflict in cmd_check: allow check for snapshot filter

reworked the code using snapshotFilter.FindAll to find all snapshots and
restic.FindUsedBlobs to retrieve all used blobs.
range repo.LookupBlob (as before) to convert the blobs to their containing packfiles
and c.repo.List(ctx, restic.PackFile, ...) to retrieve the sizes of those packfiles.

Additional documentation and tests are still outstanding.

fixed lint errors

add documentation for checking via snapshot filter.

cmd_check: change 1st line of issue-3326 to recognized format.

check with snapshot filter: changed issue description

…rt 4

Hand down filtered tree IDs to CheckWithSnapshots which builds 'usedBlobs'.
repo.LookupBlob is used to derive packfiles from 'usedBlobs' and constructs
'snapPacks' and overwrites c.packs.

…add checker_test

cmd_check, checker: added error return
checker_test: testing CheckWithSnapshots

checker_test: fixed lint error for empty tree list

based on the change in runCheck return values the following functions have been afftected
cmd/restic/cmd_check_integration_test.go
cmd/restic/cmd_migrate.go
cmd/restic/cmd_prune_integration_test.go
cmd/restic/integration_test.go

check - corrected rebase error by not removing obsolete code.

the check for the filter if len(args) > 0 { ... } is obsolete and has been replaced by
if len(args) > 0 || !opts.SnapshotFilter.Empty() { ... }

checker.go: added error message if 'selectedTrees' is empty
checker_test.go: modified test checking using test.OK() and test.Assert() to
make the checking more compact.