scmutil: introduce deprecated alias for revpair() revsingle() returns a context object, revpair() returns nodeids, revrange() returns integer revisions (in a revset). I'm going to reduce this inconsistency by making revpair() return context objects. Changing the return type is not nice to extensions, so this patch introduces a nodeid-returning version of revpair() that they can detect and use. Update callers to the new function so we can change revpair() itself and then migrate them back one by one. Differential Revision: https://phab.mercurial-scm.org/D3005

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