Fixes for diff-hunk-kill.

Fixes for diff-hunk-kill.

* lisp/vc/diff-mode.el (diff-beginning-of-hunk): Return a meaningful
value, for symmetry with diff-end-of-hunk.
(diff-split-hunk, diff-find-source-location)
(diff-ignore-whitespace-hunk, diff-refine-hunk): Use it.
(diff-bounds-of-hunk, diff-bounds-of-file): New functions.
(diff-restrict-view, diff-hunk-kill, diff-file-kill): Use them to
compute the relevant hunk or file properly.
(diff-file-junk-re): Add bzr's "modified file" tag.

Fixes: debbugs:6041 debbugs:6005