backup stats could count different-mtime, same-content files specially

[sourcefrog]

At the moment if the metadata does not match, the whole file is read and hashed (probably unavoidably) and the file is counted as modified. It might be a bit more helpful to say, if it turns out the blocks are exactly the same as in the basis version, it's a separate category. Perhaps it could be called "touched files" or more obviously "unchanged content". A similar situation will occur for permission-only changes.