Before, the scanner would could files twice if they were included in the
list of backup targets twice, e.g. `restic backup foo foo/bar` would
could the file `foo/bar` twice.
This commit uses the tree structure from the archiver to run the
scanner, so both parts see the same files.
When the scanner is slower than the actual backup, the tomb cancels the
context passed to Scan(), which then returns ctx.Err(). In the end, the
main function prints an error message that is not helpful ("Context
cancelled") and exits with an error code although no error occurred.
The code now ignores the error in the context and just uses it for
cancellation. The scanner is not supposed to return an error anyway.
Closes#1978
