Check for existing SSH ControlMaster

Be more thorough about checking for an existing ssh master by
running a test command first, and only opening up a new master
if the test fails to connect.

Change-Id: I56fe8e7b4dbc123675b7f259e81d359ed0cd55cf
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed