pull: add source information to the transaction
The source information can, should be applied once when opening the transaction
for the pull. This will lets element processed within a bundle2 be aware of them
and open the door to running a set of hooks when closing this pull transaction.
This is similar to what is done in server's unbundle call.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.