Invoke callback `cb' on each status entry
For each of the updated references, we receive a status report in the
form of ok refs/heads/master
or ng refs/heads/master <msg>
.
msg != NULL
means the reference has not been updated for the given
reason.
Return a non-zero value from the callback to stop the loop.
The callback to call on each object
0 on success, non-zero callback return value, or error code