(diff-end-of-hunk): Watch out for ambiguities.

(diff-end-of-hunk): Watch out for ambiguities.
(diff-hunk-kill): Simplify.
(diff-post-command-hook): Only apply to a single hunk.
(diff-hunk-text): Make `char-offset' non-optional.
(diff-find-text): Return a cons cell.
(diff-find-approx-text): New fun.
(diff-find-source-location): Use it.
(diff-apply-hunk, diff-test-hunk, diff-goto-source):
Adapt to new retval of diff-find-source-location.