merge.applyupdates: call driverconclude after performing merge actions

This will be a chance for the merge driver to finish resolving or generating
any driver-resolved files.

As before, having a separate error state from 'unresolved' is too big a
refactoring for now, so we hack around it by setting unresolved to a positive
value when necessary.

mercurial/merge.py

@@ -1002,6 +1002,15 b' def applyupdates(repo, actions, wctx, mc'
                 merged += 1
 
     ms.commit()
+
+    if usemergedriver and not unresolved and ms.mdstate() != 's':
+        if not driverconclude(repo, ms, wctx, labels=labels):
+            # XXX setting unresolved to at least 1 is a hack to make sure we
+            # error out
+            return updated, merged, removed, max(unresolved, 1)
+
+        ms.commit()
+
     progress(_updating, None, total=numupdates, unit=_files)
 
     return updated, merged, removed, unresolved