Luckily this one was caught while still in development as
there were more than 20 old "snapshots" to test with:
rsync: ERROR: at most 20 --link-dest args may be specified
Looking into the source:
rsync.h:151:#define MAX_BASIS_DIRS 20
There is no warning about this in the rsync manpage. This
limit should be lifted as it makes no sense to me.
The limit is now documented.