Don't allow git fetch to start ControlMaster

To avoid connectivity problems, we don't want the ssh process
that is started by git fetch to become a ControlMaster for the
overall sync task.  If it did, we would lose connectivity when
git fetch was finished with the current project, causing later
projects to not fetch efficiently.

Change-Id: I8d0dcf9b361276ff8c8b5a6324cbd4a501e9c4dd
Signed-off-by: Shawn O. Pearce <sop@google.com>

git_ssh

@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ssh -o "ControlPath $REPO_SSH_SOCK" "$@"
+exec ssh -o "ControlMaster no" -o "ControlPath $REPO_SSH_SOCK" "$@"