Fixed race condition in 'repo sync -jN' that would open multiple masters.

This fixes the SSH Control Masters to be managed in a thread-safe
fashion.  This is important because "repo sync -jN" uses threads to
sync more than one repository at the same time.  The problem didn't
show up earlier because it was masked if all of the threads tried to
connect to the same host that was used on the "repo init" line.

main.py

@@ -28,7 +28,7 @@ import re
 import sys
 
 from trace import SetTrace
-from git_config import close_ssh
+from git_config import init_ssh, close_ssh
 from command import InteractiveCommand
 from command import MirrorSafeCommand
 from command import PagedCommand
@@ -214,6 +214,7 @@ def _Main(argv):
   repo = _Repo(opt.repodir)
   try:
     try:
+      init_ssh()
       repo._Run(argv)
     finally:
       close_ssh()