listkey: display the size of the listkey payload in a debug message This is a useful information to have in general and we already have debug output related to listkeys. I'm planning to play around with massive amount of phases roots and bookmarks so having this data in debug will be very useful. This already got me to spot that one of the Logilab's review repo is exchanging 65KB of phases data during each exchanges.

ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.