merge.applyupdates: only attempt to merge files in mergeactions

This only makes a difference when a merge driver is active -- in that case we
don't want to try and merge all the files, just the ones still unresolved after
the merge driver's preprocess step is over.

mercurial/merge.py

@@ -970,7 +970,7 b' def applyupdates(repo, actions, wctx, mc'
 
     # premerge
     tocomplete = []
-    for f, args, msg in actions['m']:
+    for f, args, msg in mergeactions:
         repo.ui.debug(" %s: %s -> m (premerge)\n" % (f, msg))
         z += 1
         progress(_updating, z, item=f, total=numupdates, unit=_files)