It would be useful to be able to request "background" generation of a diff, rather than synchronously generating them and thus blocking processing.

The existing debrelease.helpers interfaces could be extended to support a flag indicating whether to operate synchronously or async.

In async mode:
- "create diff" creates a stamp file and returns "not interesting, pending, no error"
- background process walks queues of requested diffs
- diff generated in temporary location, and moved to final destination when complete
- existing checks for "is there an existing diff" need extending to "is there a pending generation request"
  - generalise out into a "diff exists" method