Many committers seem to now be using a work flow in which back-patched
commits are timestamped minutes or even hours apart in different branches
(most likely because they commit in one branch before starting work on
the next one).  git_changelog was failing to merge its reports in such
cases, so increase the max time it's willing to merge commits across.
I considered getting rid of the limit altogether, but that produces
some odd results in terms of how the merged commit gets sorted relative
to unrelated commits.