merge: tell the user when we are using bid merge
Bid merge is a new rarely used feature that the user explicitly enabled - we
should tell/warn when the user actually is using it, just like we tell when we
not are using it.
Give a message like
note: merging
3b08d01b0ab5+ and
adfe50279922 using bids from ancestors
0f6b37dbe527 and
40663881a6dd