* lisp/image/image-dired.el (image-dired-thumb-naming): New user
option to control thumbnail name.
* lisp/image/image-dired-util.el (image-dired-thumb-name): Update
to use new user option and compute contents SHA-1 if needed.
(image-dired-contents-sha1): New function to compute the SHA-1 of the
first 4KiB of a file.
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