There are three different tools that allege to do it. They all suck. I have been editing RCS files by hand, with a paper clip and a nine volt battery. Oh, the indignity.
<jwz:~/www/> git whatchanged index.html | tail -8
Author: Jamie Zawinski <firstname.lastname@example.org>
Date: Thu Oct 13 19:10:38 1994 -0700
:000000 100644 0000000... c28db08... A index.html
"Look, Ben! All of man's past glory -- in one large SAND PIT!"
Well played, Apple. Well played.
I let it run for a while and it actually got up to "33 of 2.5 MB" before I gave up. It did the same thing after killing and restarting Mail.
The raw message was, in fact, 2.5 MB (for a 1.9 MB JPEG).