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

[Ivan Shmakov]

>>>>> Stefan Monnier <monnier@iro.umontreal.ca> writes:

 >>> 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.

        I have no reason to insist on doing it either way.  However, it
        still feels inconsistent to check for a specific option, and
        then use its alias.

        Then, however, there’s the following in diffutils/NEWS:

 > User-visible changes in version 2.8:

[…]

 > * The following diff options are still accepted, but are no longer 
 > documented.
 >   They may be withdrawn in future releases.
 >   -h (omit; it has no effect)
 >   -H (use --speed-large-files instead)
 >   -L (use --label instead)

        (Version 2.8 was apparently released March, 2002.)

 > 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).

        Again, I have no reason to argue for retaining the check,
        especially given that none of my systems seem to have diff(1)
        versions other than the one from Diffutils, as of 3.0, or a
        later version.

        Also to note:

 > User-visible changes in version 2.0:

[…]

 > * diff options renamed:
 > --label renamed from --file-label

        Now, given that 2.7 – the oldest version available from
        http://ftp.gnu.org/gnu/diffutils/ – dates back to 1994…

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