Automatically use SSH control master support during sync

By creating a background ssh "control master" process which lives
for the duration of our sync cycle we can easily cut the time for
a no-op sync of 132 projects from 60s to 18s.

Bug: REPO-11
Signed-off-by: Shawn O. Pearce <sop@google.com>

main.py

@@ -28,6 +28,7 @@ import re
 import sys
 
 from trace import SetTrace
+from git_config import close_ssh
 from command import InteractiveCommand
 from command import MirrorSafeCommand
 from command import PagedCommand
@@ -212,7 +213,10 @@ def _Main(argv):
 
   repo = _Repo(opt.repodir)
   try:
-    repo._Run(argv)
+    try:
+      repo._Run(argv)
+    finally:
+      close_ssh()
   except KeyboardInterrupt:
     sys.exit(1)
   except RepoChangedException, rce: