(tar-swap-data): New function.

(tar-swap-data): New function.
(tar-change-major-mode-hook): Use it instead of buffer-swap-text.
(tar-mode-revert): Likewise.
(tar-mode): Likewise.  Don't set write-region-annotate-functions.
(tar-data-swapped-p): Test buffer-swapped-with.
(tar-write-region-annotate): Remove.
(tar-header-block-tokenize): Add argument `coding' and use it
instead of tar-file-name-coding-system.
(tar-summarize-buffer): Pass tar-file-name-coding-system here.