As we already walk the history of the branch that gets merged to
come up with a short log, let's label it with names of the primary
authors, so that the user who summarizes the merge can easily give
credit to them in the log message.
Also infer the names of "lieutents" to help integrators at higher
level of the food-chain to give credit to them, by counting:
* The committer of the 'tip' commit that is merged
* The committer of merge commits that are merged
Often the first one gives the owner of the history being pulled, but
his last pull from his sublieutenants may have been a fast-forward,
in which case the first one would not be. The latter rule will
count the integrator of the history, so together it might be a
reasonable heuristics.
Signed-off-by: Junio C Hamano <gitster@pobox.com>