(pre) push should check for new revisions

[gaak99]

The user should be warned if a new revision was saved on Dropbox before a push is done.

[gaak99]

Stepping back a bit ...

The current code has data races galore -- was not orig designed to be robust if mult users at once -- but if we now can make it so would be nice...

Would be xtra nice if we had a way (api, lock file? aka file.lck) to upload the new version and update ancdb atomically -- but only if here there we're no changes to data file or ancdb since we last fetched it. If it detects a change then it errors out.

In lieu of that, we should probably replace one big ancdb with a file.anc per data file --
thus limit the collisions if working with mult files.