Add a comment explaining why readline is the only way we can avoid buffering

tools/valgrind/common.py

Index: tools/valgrind/common.py
===================================================================
--- tools/valgrind/common.py	(revision 202543)
+++ tools/valgrind/common.py	(working copy)
@@ -55,6 +55,8 @@
   if timeout > 0:
     wait_until = time.time() + timeout
   while p.poll() is None and not did_timeout:
+    # Have to use readline rather than readlines() or "for line in p.stdout:",
+    # otherwise we get buffered even with bufsize=0.
     line = p.stdout.readline()
     while line and not did_timeout:
       sys.stdout.write(line)