bug#18824: split diff-check-labels off diff-no-select

[Lars Ingebrigtsen]

Ivan Shmakov <ivan@siamics.net> writes:

> Package:  emacs
> Severity: wishlist
> Tags:     patch
>
>       Please provide a separate diff-check-labels function to allow
>       for the code calling diff.el facilities to supply its own
>       --label= arguments to diff in place of diff.el-generated ones.
>
>       Possible patch, as well as the example usage, are MIMEd.

I think this patch looks fine.  Could you also provide a NEWS entry?

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