bug#18850: smerge-mode: use diff-check-labels

[Stefan Monnier]

>> That looks OK, yes.  Tho you don't need to change -L to --label.
>       The point is that, strictly speaking, (diff-check-labels) only
>       checks for the --label option proper, not for its aliases.

Of course, part of the question, then is: are there diff commands that
handle one of the two but not the other.  If there are, then we should
use the one that's more often supported, and if there aren't then it
doesn't matter.
So far, smerge-mode hasn't even bothered to check, so maybe the check
isn't that important any more (IIRC it was added for Solaris's diff,
tho I'd suspect that current derivatives of Solaris woujld accept
either -L or --label or both).


        Stefan