merge.applyupdates: call driverpreprocess before starting merge actions

We also need to update our internal state to whatever state driverpreprocess
leaves it in.

Adding an error state separate from the unresolved count is too big a
refactoring for now, so we hack around it by setting it to a positive value to
indicate an error state.

mercurial/merge.py

@@ -942,6 +942,27 b' def applyupdates(repo, actions, wctx, mc'
         util.setflags(repo.wjoin(f), 'l' in flags, 'x' in flags)
         updated += 1
 
+    mergeactions = actions['m']
+    # the ordering is important here -- ms.mergedriver will raise if the merge
+    # driver has changed, and we want to be able to bypass it when overwrite is
+    # True
+    usemergedriver = not overwrite and mergeactions and ms.mergedriver
+
+    if usemergedriver:
+        ms.commit()
+        proceed = driverpreprocess(repo, ms, wctx, labels=labels)
+        # the driver might leave some files unresolved
+        unresolvedf = set(ms.unresolved())
+        if not proceed:
+            # XXX setting unresolved to at least 1 is a hack to make sure we
+            # error out
+            return updated, merged, removed, max(len(unresolvedf), 1)
+        newactions = []
+        for f, args, msg in mergeactions:
+            if f in unresolvedf:
+                newactions.append((f, args, msg))
+        mergeactions = newactions
+
     # premerge
     tocomplete = []
     for f, args, msg in actions['m']: