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>

git_ssh

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec ssh -o "ControlPath $REPO_SSH_SOCK" "$@"