lock: loop a finite number of times in trylock (issue4787)

..before falling back to lock, which will sleep. This only addresses
the CPU consumption of the reported issue.

mercurial/lock.py

@@ -83,7 +83,9 @@ class lock(object):
         if lock._host is None:
             lock._host = socket.gethostname()
         lockname = '%s:%s' % (lock._host, self.pid)
-        while not self.held:
+        retry = 5
+        while not self.held and retry:
+            retry -= 1
             try:
                 self.vfs.makelock(lockname, self.f)
                 self.held = 1