Repo: improve error detection for new ssh connections

this check can only detect errors that happen within 1 sec after launching
ssh. But this is typically enough to catch configuration issues like
'connection refused' or 'authentication failed'.

Change-Id: I00b6f62d4c2889b1faa6c820e49a198554c92795

git_config.py

@@ -464,9 +464,13 @@ def _open_ssh(host, port=None):
              % (host,port, str(e)), file=sys.stderr)
       return False
 
+    time.sleep(1)
+    ssh_died = (p.poll() is not None)
+    if ssh_died:
+      return False
+
     _master_processes.append(p)
     _master_keys.add(key)
-    time.sleep(1)
     return True
   finally:
     _master_keys_lock.release()