l33t pointed out that currently, Hg-Git doesn't provide any confirmation that a
push was successful other than the exit code.  Normal Mercurial provides a
couple other messages followed by "added X changesets with Y changes to
Z files".  After this change, Hg-Git will provide much more similar output.
It's not identical, as the underlying model is substantially different, but the
concept is the same.  The main message is "added X commits with Y trees and
Z blobs".

This change doesn't affect the output of what references/branches were touched.
That will be addressed in a subsequent commit.

Dulwich doesn't provide an easy hook to get the information needed for this
output.  Instead of passing generate_pack_contents as the pack generator
function to send_pack, I pass a custom function that determines the "missing"
objects, stores the counts, and then calls generate_pack_contents (which then
will determine the "missing" objects again.

The new expected output:
searching for changes # unless quiet true
<N> commits found     # if verbose true
list of commits:      # if debugflag true and at least one commit found
<each hash>           # if debugflag true and at least one commit found
adding objects        # if at least one commit found unless quiet true
added <N> commits with <N> trees and <N> blobs # if at least one object unless
                                               # quiet true

https://bitbucket.org/durin42/hg-git/issue/64/push-confirmation