Data loss on fork over moved file

[madadam]

Consider two replicas A and B:

1. A creates a file
2. B syncs and merges with A
3. Concurrently:
   • A renames / moves the file
   • B updates the file
4. A syncs with B and starts to merge

The merge proceeds normally because there is no corresponding entry in A's branch (there is only a tombstone after the file was moved). However, as merge preserves blob ids, the merge begins to overwrite the renamed file, resulting it data loss.

Ideas for fix:

• When this situation occurs, generate a new blob_id for the dst file of the merge
• Always generate new blob_id on move/rename
• Something else ...

Related to #70