Skip sleep and retry if git remote update exits with a signal

Pressing ctrl-c during repo sync often hangs for 30 to 45 seconds
due to the time.sleep and retry in _RemoteFetch.  If git exits with
a signal, for example -2 for SIGINT triggered by ctrl-c, skip the
sleep and retry.

Change-Id: I32da12c2dcc96d9cc0b12a066e824b12ebfb52a0