addbranchrevs: no longer accept revset as "revs" (API)

The only caller was removed in 0b4692b9646d (bundle: avoid
repo.lookup() for converting revnum to nodeid, 2018-04-02).

Differential Revision: https://phab.mercurial-scm.org/D3192

mercurial/hg.py

@@ -66,9 +66,7 b' def addbranchrevs(lrepo, other, branches'
     hashbranch, branches = branches
     if not hashbranch and not branches:
         x = revs or None
-        if util.safehasattr(revs, 'first'):
-            y = revs.first()
-        elif revs:
+        if revs:
             y = revs[0]
         else:
             y = None