[Svnmerge] [PATCH] performance fix for some platforms

[lsuvkne at onemodel.org]

I forgot the suggested changelog comment:

[[[
Uses new subprocess module for in launch(), when python version allows, for a 
big performance benefit on some operating systems.  Faster because doesn't 
try to close all (unused) file descriptors. 

* contrib/client-side/svnmerge/svnmerge.py
(launch):  Use subprocess module when possible for better performance.
* contrib/client-side/svnmerge/svnmerge_test.py
(multilaunch):  No longer passing empty cmd strings.
]]]