The problem is similar to a7bf0311 (wcfs: Fix crash if on invalidation
handledδZ needs to access ZODB) - I forgot to put zhead's transaction into
context.

Without the fix added test fails as:

    wcfs_test.py::test_wcfs_crash_old_data
    ---------------- live log call -----------------
    WARNING  ZODB.FileStorage:FileStorage.py:413 Ignoring index for /tmp/testdb_fs.OV0rS6/1.fs

    M: commit -> @at0 (03e5a3342bc5ab22)

    M: commit -> @at1 (03e5a3342bc88899)
    M:      f<0000000000000002>     [0]
    INFO     wcfs:__init__.py:293 starting for file:///tmp/testdb_fs.OV0rS6/1.fs ...
    I0120 17:12:10.274379  704327 wcfs.go:2393] start "/dev/shm/wcfs/556fa61a9f9675f34c6b44e1f978842c37176c59" "file:///tmp/testdb_fs.OV0rS6/1.fs"
    I0120 17:12:10.274409  704327 wcfs.go:2399] (built with go1.17.6)
    W0120 17:12:10.274560  704327 storage.go:152] zodb: FIXME: open file:///tmp/testdb_fs.OV0rS6/1.fs: raw cache is not ready for invalidations -> NoCache forced
    INFO     wcfs:__init__.py:334 started pid704327 @ /dev/shm/wcfs/556fa61a9f9675f34c6b44e1f978842c37176c59

    C: setup watch f<0000000000000002> @at1 (03e5a3342bc88899)
    #  pinok: {}

    M: commit -> @at2 (03e5a3342c895777)
    M:      f<0000000000000002>     [1]

    M: commit -> @at3 (03e5a3342ca5ef55)
    M:      f<0000000000000002>     [0]

    C: setup watch f<0000000000000002> @at2 (03e5a3342c895777)
    #  pinok: {0: @at1 (03e5a3342bc88899)}
    panic: transaction: no current transaction

    goroutine 88 [running]:
    lab.nexedi.com/kirr/neo/go/transaction.currentTxn({0x969718, 0xc0000b6240})
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/transaction.go:59 +0x77
    lab.nexedi.com/kirr/neo/go/transaction.Current(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/transaction/api.go:206
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).checkTxnCtx(...)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:374
    lab.nexedi.com/kirr/neo/go/zodb.(*Connection).Get(0xc0000c25a0, {0x969718, 0xc0000b6240}, 0x4)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/connection.go:331 +0x73
    lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata.(*ΔFtail).BlkRevAt(0xc00009dd40, {0x969718, 0xc0000b6240}, 0xc000100540, 0x30, 0x3e5a3342c895777)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/zdata/δftail.go:1140 +0x39d
    main.(*WatchLink).setupWatch(0xc0000120a0, {0x969718, 0xc0000b6240}, 0x2, 0x3e5a3342c895777)
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1754 +0xe3f
    main.(*WatchLink)._handleWatch(0x0, {0x969718, 0xc0000b6240}, {0xc0000a0122, 0x0})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1973 +0x65
    main.(*WatchLink).handleWatch(0x0, {0x969718, 0xc0000b6240}, 0x0, {0xc0000a0122, 0x28})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1955 +0x10c
    main.(*WatchLink)._serve.func3({0x969718, 0xc0000b6240})
            /home/kirr/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/wcfs.go:1944 +0x3c
    lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go.func1()
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:86 +0x68
    created by lab.nexedi.com/kirr/go123/xsync.(*WorkGroup).Go
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/xsync/xsync.go:83 +0x92
    >>> Change history by file:

    f<0000000000000002>:
                                    0 1 2 3 4 5 6 7
                                    a b c d e f g h
            @at0 (03e5a3342bc5ab22)
            @at1 (03e5a3342bc88899) 0
            @at2 (03e5a3342c895777)   1
            @at3 (03e5a3342ca5ef55) 0

    ----------------------------------------

            # wcfs was crashing in setting up watch because of "1" and "2" from above, and
            # 3. setupWatch was calling ΔFtail.BlkRevAt without putting zhead's transaction into ctx.
            wl2 = t.openwatch()
    >       wl2.watch(zf, at2, {0:at1})