(diff-default-read-only): Change default.

(diff-default-read-only): Change default.
(diff-mode-hook): Make it a defcustom.  Add some options.
(diff-mode-map): Bind diff-refine-hook.
(diff-yank-handler): New var.
(diff-yank-function): New fun.
(diff-font-lock-keywords): Use them.
(diff-end-of-file): Handle case where file-header looks like diff text.
(diff-hunk-kill): Adjust to "new" hunk-next behavior.
(diff-file-kill): Delete a subsequent empty line, if applicable.
(diff-hunk-file-names): New fun, extracted from diff-tell-file-name.
(diff-find-file-name): Use it.
(diff-tell-file-name): New command.
(diff-mode): Be careful with view-mode.
(diff-delete-if-empty, diff-delete-empty-files, diff-make-unified):
New functions, for use in diff-mode-hook.
(diff-find-source-location): Catch "regex too large" errors.
(diff-apply-hunk, diff-test-hunk): Go to old or new file.
(diff-refine-hunk): New command.