verify: hold write lock

Verify will report spurious errors if commits or pulls happen during verify.

mercurial/verify.py

@@ -17,6 +17,8 b' def verify(repo):'
     warnings = [0]
     neededmanifests = {}
 
+    lock = repo.lock()
+
     def err(msg):
         repo.ui.warn(msg + "\n")
         errors[0] += 1