Simon,
  chances are this has nothing to do with rebasing but rather with the
anti-virus product on your system.
Do you happen to use Comodo CIS? Without proper configuration, Cygwin
will show such fork fails in some situations.

Cheers,
--J.

On 03/04/2018 17:20, Simon Matthews wrote:
>> I have been seeing a large number of error messages like this:
>> 2 [main] bash 8652 fork: child -1 - forked process 7532 died
>> unexpectedly, retry 0, exit code 0xC0000005, errno 11
>> 2 [main] bash 8652 fork: child -1 - forked process 7532 died
>> unexpectedly, retry 0, exit code 0xC0000005, errno 11
>>
>> [...]