fw-headless stuck with unpushed hg commits

[myieye]

Describe the bug
#2057 resulted in hg commits in fw-headless that were not pushed to the Lexbox repo.

There was no way to trigger a Send/Receive without making real modifications to at least on version of the project.
(The first Send/Receive in the sync process checks if there are new commits in Lexbox, which there weren't and the send Send/Receive checks if the sync caused any fwdata changes, which it didn't.)

This is obviously an error case, but if we're going to survive hundreds of syncs, we need to find ways to automatically recover from stuff like this.

Potential improvement for manual recoveries:
We might want an endpoint that can be used to force a Send/Receive.
(I recently merged a new endpoint that can be used to only do the first part of a sync / almost only do a Harmony Sync.)

Potential improvement for automatic recovery:
Perhaps the second Send/Receive should not only check for fwdata changes, but also for outgoing commits.
I don't really want the first Send/Receive to trigger in this case. I think I'd like a full sync to complete successfully before a new one is started.