Make it more robust in the presence of empty context lines in unified hunks.

Make it more robust in the presence of empty context lines in unified hunks.
(diff-valid-unified-empty-line): New var.
(diff-unified->context, diff-sanity-check-hunk): Obey it.
(diff-end-of-hunk): Obey it.  New arg `donttrustheader'.
(diff-fixup-modifs, diff-post-command-hook): Use this new arg.
(diff-hunk-header-re-unified): New const.
(diff-font-lock-keywords, diff-hunk-header-re, diff-split-hunk)
(diff-fixup-modifs, diff-unified->context, diff-next-complex-hunk)
(diff-sanity-check-hunk): Use it.