bug#25410: 26.0.50; Refine an unified diff hunk only if adds lines

[npostavs]

Tino Calancha <tino.calancha@gmail.com> writes:

> After deletion of a large file from CVS, a diff shows
> a very large hunk with just deleted lines.  Then, for unified diffs, a call
> to `diff-refine-hunk' on that hunk takes a huge time.
> Instead, it's better to first check if the hunk adds new lines: only when
> this is true, then proceed with the hunk refinement.

What about a diff that adds a very large file?  Perhaps we should only
refine if there added lines *and* deleted lines?