'ZBlk0' and 'ZBlk1' always only change the block that's explicitly altered,
while zblk format 'auto' may also alter blocks different to the explicitly
changed blocks. In other words, as @kirr noted in nexedi/wendelin.core!20 (comment 198863),
format 'auto' break the invariant

    "transaction changes ZBlk objects only for modified blocks of array" [1]

Breaking this invariant leads to two failed WCFS tests -
'test_wcfs_crash_old_data' and 'test_wcfs_watch_vs_access'.

As suggested in nexedi/wendelin.core!20 (comment 198863)
instead of patching tests, we rather try not to break this invariant
with format 'auto'. This is exactly what this patch does.

[1] nexedi/wendelin.core!20 (comment 198863)