Now that git does pretty reliable date parsing, we might as well get
the date from the email itself. Of course, it's still questionable
whether the date on the email is all that relevant, but it's certainly
no worse than taking the commit date.
I looked a bit at my old BK tools for the same thing, but they were
just so horrid in many ways that I largely rewrote it all and these
tools do things a bit differently. Instead of aggressively piping
data from one process to another (which was clever but very hard
to follow), this first just splits out the mbox into many smaller
email files, and then does some scripts on these temporary files.
