largefiles: put whole rollback-ing process into the same "wlock" scope

Before this patch, there are three distinct "wlock" scopes in
"overriderollback":

  1. "localrepository.rollback" via original "rollback" command,
  2. "merge.update" for reverting standin files only, and
  3. "overriderollback" specific "wlock" scope (to synchronize
     largefile dirstate)

But these should be executed in the same "wlock" scope for
consistency.

hgext/largefiles/overrides.py

@@ -1140,11 +1140,12 b' def overridepurge(orig, ui, repo, *dirs,'
     repo.status = oldstatus
 
 def overriderollback(orig, ui, repo, **opts):
-    result = orig(ui, repo, **opts)
-    merge.update(repo, node=None, branchmerge=False, force=True,
-        partial=lfutil.isstandin)
     wlock = repo.wlock()
     try:
+        result = orig(ui, repo, **opts)
+        merge.update(repo, node=None, branchmerge=False, force=True,
+                     partial=lfutil.isstandin)
+
         lfdirstate = lfutil.openlfdirstate(ui, repo)
         lfiles = lfutil.listlfiles(repo)
         oldlfiles = lfutil.listlfiles(repo, repo[None].parents()[0].rev())