return an aggregate value from Contents.merge

[gregtatcam]

It would be nice to have an aggregate value returned from Contents.merge. For instance in IMAP application when two email databases are merged a mailbox statistics needs to be updated after the merge.

[samoht]

An idea would be to pass an accumulator to the merge function. Not totally sure how to expose this though. Could be part of a larger context, where you'd want to expose more things that just the paths (as it is the case actually) -- other stuff could be the commits being merged, etc..