util: allow to specify buffer size in popen4

We will need unbuffered IO to restore real time output with ssh peer.

Changeset ec171737aaf1 seems to indicate playing with this value could be
dangerous, but does not indicate why.

mercurial/util.py

@@ -247,8 +247,8 b' def popen3(cmd, env=None, newlines=False'
     stdin, stdout, stderr, p = popen4(cmd, env, newlines)
     return stdin, stdout, stderr
 
-def popen4(cmd, env=None, newlines=False):
-    p = subprocess.Popen(cmd, shell=True, bufsize=-1,
+def popen4(cmd, env=None, newlines=False, bufsize=-1):
+    p = subprocess.Popen(cmd, shell=True, bufsize=bufsize,
                          close_fds=closefds,
                          stdin=subprocess.PIPE, stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE,