merge: check that there are no conflicts after --abort

Same idea as in abcc82bf0717 (clean: check that there are no conflicts
after, 2020-01-24). We should reuse more code here, but that will come
later.

Differential Revision: https://phab.mercurial-scm.org/D7994

mercurial/hg.py

@@ -1183,8 +1183,8 b' def abortmerge(ui, repo):'
 
     repo.ui.status(_(b"aborting the merge, updating back to %s\n") % node[:12])
     stats = mergemod.update(repo, node, branchmerge=False, force=True)
+    assert stats.unresolvedcount == 0
     _showstats(repo, stats)
-    return stats.unresolvedcount > 0
 
 
 def _incoming(