Allow sync to run even when the manifest is broken.

If the current manifest is broken then "repo sync" fails because it
can't retrieve the default value for --jobs. Use 1 in this case, in
order that you can "repo sync" to get a fixed manifest (assuming someone
fixed it upstream).

Change-Id: I4262abb59311f1e851ca2a663438a7e9f796b9f6