util.system: avoid buffering of subprocess output when it is piped

util.system() copies subprocess' output through pipe if output file is not
stdout.  Because a file iterator has internal buffering, output won't be
flushed until enough data is available.  Therefore, it could easily miss
important messages such as "waiting for lock".

mercurial/util.py

@@ -649,7 +649,10 @@ def system(cmd, environ={}, cwd=None, onerr=None, errprefix=None, out=None):
             proc = subprocess.Popen(cmd, shell=True, close_fds=closefds,
                                     env=env, cwd=cwd, stdout=subprocess.PIPE,
                                     stderr=subprocess.STDOUT)
-            for line in proc.stdout:
+            while True:
+                line = proc.stdout.readline()
+                if not line:
+                    break
                 out.write(line)
             proc.wait()
             rc = proc.returncode