database is older than the current git-annex branch. Diff from old to
new branch to efficiently update.
+ Note ideas in above todo about doing this at git-annex branch merge
+ time to reuse the git diff done there.
+
+ * What if 2 concurrent threads are considering sending two different
+ keys to a repo at the same time. It can hold either but not both.
+ It should avoid sending both in this situation. (Also discussed in
+ above todo)
+
* `fullybalanced=foo:2` can get stuck in suboptimal situations. Eg,
when 2 out of 3 repositories are full, and the 3rd is mostly empty,
it is no longer possible to add new files to 2 repositories.